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

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