doc: document subpath-pattern precedence and null blocking in exports

The exports resolution sorts subpath patterns by specificity
(lib/internal/modules/esm/resolve.js patternKeyCompare), so the pattern
with the longest matching prefix wins regardless of declaration order.
A null target blocks any subpath matching its pattern, regardless of key
position. The documentation did not state either rule, so add a short
clarification after the existing null example.

Fixes: #44517

Author: Phantom-d-e-v

doc/api/packages.md

@@ -655,6 +655,15 @@ import featureX from 'es-module-package/features/x.js';
 // Loads ./node_modules/es-module-package/src/features/x.js
 ```
 
+When a subpath matches more than one pattern, the pattern with the longest
+matching prefix (the most specific pattern) takes precedence, regardless of the
+order in which the patterns are declared in the `exports` object.
+
+A `null` target blocks access to the subpaths that would otherwise match its
+pattern: requesting such a subpath throws
+[`ERR_PACKAGE_PATH_NOT_EXPORTED`][] regardless of where the `null` pattern is
+positioned among the keys.
+
 ### Conditional exports
 
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