Document same-library access for protected members

I noticed that in the Flutter framework, `@protected` members are called from outside of instance methods. @Hixie responded:

> protected allows access from the same file

I think this should be documented.

Closes https://github.com/dart-lang/sdk/pull/45937
https://github.com/dart-lang/sdk/pull/45937

GitOrigin-RevId: c1a86d4039aeaa5a2126b505cad28e10fe361b0a
Change-Id: If0be693897abe10197516bdf854ec45fcb1be7fc
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/198480
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Brian Wilkerson <brianwilkerson@google.com>
diff --git a/pkg/meta/lib/meta.dart b/pkg/meta/lib/meta.dart
index c6dff3b..45b2e84 100644
--- a/pkg/meta/lib/meta.dart
+++ b/pkg/meta/lib/meta.dart
@@ -209,9 +209,10 @@
 /// field) `m` in a class `C`. If the annotation is on a field it applies to the
 /// getter, and setter if appropriate, that are induced by the field. Indicates
 /// that `m` should only be invoked from instance methods of `C` or classes that
-/// extend, implement or mix in `C`, either directly or indirectly. Additionally
-/// indicates that `m` should only be invoked on `this`, whether explicitly or
-/// implicitly.
+/// extend, implement or mix in `C`, either directly or indirectly or from within
+/// the same library that `m` is declared in. Additionally indicates that `m`
+/// should only be invoked on `this` (when outside of the library that it is
+/// declared in), whether explicitly or implicitly.
 ///
 /// Tools, such as the analyzer, can provide feedback if
 ///