docs(auth): update account-exists guidance after fetchSignInMethodsForEmail removal

packages/firebase_auth/firebase_auth/lib/src/firebase_auth.dart

@@ -466,12 +466,10 @@ class FirebaseAuth extends FirebasePlugin implements FirebaseService {
   /// A [FirebaseAuthException] maybe thrown with the following error code:
   /// - **account-exists-with-different-credential**:
   ///  - Thrown if there already exists an account with the email address
-  ///    asserted by the credential.
-  // ignore: deprecated_member_use_from_same_package
-  ///    Resolve this by asking
-  ///    the user to sign in using one of the returned providers.
-  ///    Once the user is signed in, the original credential can be linked to
-  ///    the user with [linkWithCredential].
+  ///    asserted by the credential. Resolve this by asking the user to sign in
+  ///    with a provider already linked to that email address. Once the user is
+  ///    signed in, the original credential can be linked to the user with
+  ///    [linkWithCredential].
   /// - **invalid-credential**:
   ///  - Thrown if the credential is malformed or has expired.
   /// - **operation-not-allowed**:

packages/firebase_auth/firebase_auth_platform_interface/lib/src/auth_credential.dart

@@ -23,8 +23,7 @@ class AuthCredential {
   final String providerId;
 
   /// The authentication sign in method for the credential. For example,
-  /// 'password', or 'emailLink'. This corresponds to the sign-in method
-  /// identifier returned in [fetchSignInMethodsForEmail].
+  /// 'password', or 'emailLink'.
   final String signInMethod;
 
   /// A token used to identify the AuthCredential on native platforms.

packages/firebase_auth/firebase_auth_platform_interface/lib/src/platform_interface/platform_interface_firebase_auth.dart

@@ -458,10 +458,10 @@ abstract class FirebaseAuthPlatform extends PlatformInterface {
   /// A [FirebaseAuthException] maybe thrown with the following error code:
   /// - **account-exists-with-different-credential**:
   ///  - Thrown if there already exists an account with the email address
-  ///    asserted by the credential. Resolve this by calling
-  ///    [fetchSignInMethodsForEmail] and then asking the user to sign in using
-  ///    one of the returned providers. Once the user is signed in, the original
-  ///    credential can be linked to the user with [linkWithCredential].
+  ///    asserted by the credential. Resolve this by asking the user to sign in
+  ///    with a provider already linked to that email address. Once the user is
+  ///    signed in, the original credential can be linked to the user with
+  ///    [linkWithCredential].
   /// - **invalid-credential**:
   ///  - Thrown if the credential is malformed or has expired.
   /// - **operation-not-allowed**:

packages/firebase_auth/firebase_auth_platform_interface/lib/src/platform_interface/platform_interface_user.dart

@@ -181,9 +181,9 @@ abstract class UserPlatform extends PlatformInterface {
   ///    user, an `email` and `credential` ([AuthCredential]) fields are also
   ///    provided. You have to link the credential to the existing user with
   ///    that email if you wish to continue signing in with that credential.
-  ///    To do so, call [fetchSignInMethodsForEmail], sign in to `email` via one
-  ///    of the providers returned and then [User.linkWithCredential] the
-  ///    original credential to that newly signed in user.
+  ///    To do so, sign in to `email` with a provider already linked to it, then
+  ///    [User.linkWithCredential] the original credential to that newly signed
+  ///    in user.
   /// - **operation-not-allowed**:
   ///  - Thrown if you have not enabled the provider in the Firebase Console. Go
   ///    to the Firebase Console for your project, in the Auth section and the
@@ -227,9 +227,9 @@ abstract class UserPlatform extends PlatformInterface {
   ///    user, an `email` and `credential` ([AuthCredential]) fields are also
   ///    provided. You have to link the credential to the existing user with
   ///    that email if you wish to continue signing in with that credential.
-  ///    To do so, call [fetchSignInMethodsForEmail], sign in to `email` via one
-  ///    of the providers returned and then [User.linkWithCredential] the
-  ///    original credential to that newly signed in user.
+  ///    To do so, sign in to `email` with a provider already linked to it, then
+  ///    [User.linkWithCredential] the original credential to that newly signed
+  ///    in user.
   /// - **operation-not-allowed**:
   ///  - Thrown if you have not enabled the provider in the Firebase Console. Go
   ///    to the Firebase Console for your project, in the Auth section and the
@@ -329,9 +329,9 @@ abstract class UserPlatform extends PlatformInterface {
   ///    user, an `email` and `credential` ([AuthCredential]) fields are also
   ///    provided. You have to link the credential to the existing user with
   ///    that email if you wish to continue signing in with that credential.
-  ///    To do so, call [fetchSignInMethodsForEmail], sign in to `email` via one
-  ///    of the providers returned and then [User.linkWithCredential] the
-  ///    original credential to that newly signed in user.
+  ///    To do so, sign in to `email` with a provider already linked to it, then
+  ///    [User.linkWithCredential] the original credential to that newly signed
+  ///    in user.
   /// - **operation-not-allowed**:
   ///  - Thrown if you have not enabled the provider in the Firebase Console. Go
   ///    to the Firebase Console for your project, in the Auth section and the
@@ -368,9 +368,9 @@ abstract class UserPlatform extends PlatformInterface {
   ///    user, an `email` and `credential` ([AuthCredential]) fields are also
   ///    provided. You have to link the credential to the existing user with
   ///    that email if you wish to continue signing in with that credential.
-  ///    To do so, call [fetchSignInMethodsForEmail], sign in to `email` via one
-  ///    of the providers returned and then [User.linkWithCredential] the
-  ///    original credential to that newly signed in user.
+  ///    To do so, sign in to `email` with a provider already linked to it, then
+  ///    [User.linkWithCredential] the original credential to that newly signed
+  ///    in user.
   /// - **operation-not-allowed**:
   ///  - Thrown if you have not enabled the provider in the Firebase Console. Go
   ///    to the Firebase Console for your project, in the Auth section and the