SERVER-9516 Fix concurrency error around AuthorizationManager::CacheGuard::endFetchPhase().

That method should not clear the _isThisGuardInFetchPhase field.  Rather, the
fassert ensuring preconditions for isSameCacheGeneration() should recognize the
true "post fetch phase" condition, that _isThisGuardInFetchPhase is true and
that _lock.owns_lock() is also true.
SERVER-9516 Fix fassert number, add missing comment.