doc: git rebase: clarify arguments syntax

Remove duplicate explanation of `git rebase <upstream> <branch>` which
is already explained above.

Signed-off-by: Julia Evans <julia@jvns.ca>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: jvns

Documentation/git-rebase.adoc

@@ -62,11 +62,7 @@ one of these things:
 
    git rebase --skip
 
-If `<branch>` is specified, `git rebase` will perform an automatic
-`git switch <branch>` before doing anything else.  Otherwise
-it remains on the current branch.
-
-If `<upstream>` is not specified, the upstream configured in
+If you don't specify an `<upstream>` to rebase onto, the upstream configured in
 `branch.<name>.remote` and `branch.<name>.merge` options will be used (see
 linkgit:git-config[1] for details) and the `--fork-point` option is
 assumed.  If you are currently not on any branch or if the current