docs(gotrue): document the _refreshTokenCompleter concurrent-request deduplication pattern (#1342)

Expand the comment on `_refreshTokenCompleter` to explain the race
condition it prevents, how the deduplication mechanism works, and
that the completer is cleared after resolution for the next cycle.

Linear: SDK-813

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: grdsdev

packages/gotrue/lib/src/gotrue_client.dart

@@ -56,7 +56,13 @@ class GoTrueClient {
 
   Timer? _autoRefreshTicker;
 
-  /// Completer to combine multiple simultaneous token refresh requests.
+  /// Deduplicates concurrent token refresh requests.
+  ///
+  /// When multiple callers detect an expired token simultaneously (e.g. several
+  /// in-flight API requests all fail with 401), only the first creates a new
+  /// refresh network request. Subsequent callers receive the same [Future] and
+  /// wait for the single in-flight request to complete. The completer is cleared
+  /// after resolution so the next expiry cycle starts a fresh request.
   Completer<AuthResponse>? _refreshTokenCompleter;
 
   JWKSet? _jwks;