add comment about b/388888512

Author: dconeybe

Firestore/core/src/util/string_format.h

@@ -66,6 +66,11 @@ class FormatArg final : public absl::AlphaNum {
  public:
   template <typename T>
   FormatArg(T&& value,
+    // TODO(b/388888512) Remove the usage of StringifySink since it is not part
+    //  of absl's public API. Moreover, subclassing AlphaNum is not supported
+    //  either, so find a way to do this without these two caveats.
+    //  See https://github.com/firebase/firebase-ios-sdk/pull/14331 for a
+    //  partial proposal.
             absl::strings_internal::StringifySink&& sink =
                 {})  // NOLINT(runtime/explicit)
       : FormatArg{std::forward<T>(value), std::move(sink),