[dart:js_interop] Improve globalContext documentation

Fixes https://github.com/dart-lang/sdk/issues/60900

"globalContext.name" is confusing because it implies that's
the code it's executing rather than getting the property
'name' off of the globalContext getter.

Also adds a library annotation with no @JS annotation in the
example to make it clear there's no library renaming (because
otherwise we'd be fetching the rename first off of globalContext).

CoreLibraryReviewExempt: Doc-only change.
Change-Id: I77cac4b9fd7c29f06c1c579879126031ad6b8dac
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/434523
Reviewed-by: Erik Ernst <eernst@google.com>
Commit-Queue: Srujan Gaddam <srujzs@google.com>

sdk/lib/js_interop/js_interop.dart

@@ -1388,11 +1388,14 @@ extension JSAnyOperatorExtension on JSAny? {
 /// For example:
 ///
 /// ```
+/// library;
+///
 /// @JS()
 /// external String get name;
 /// ```
 ///
-/// Reading `name` will execute JavaScript code like `globalContext.name`.
+/// Reading the top-level member `name` will execute JavaScript code like
+/// `<globalContext>.name`.
 ///
 /// There are subtle differences depending on the compiler, but in general,
 /// [globalContext] can be treated like JavaScript's `globalThis`.