Improve comments about `absl::optional<T>` support.

Change-Id: I84753d59d021b0953cd3f6c96c0a96574a7f4e6c
Reviewed-on: https://code-review.googlesource.com/c/re2/+/62030
Reviewed-by: Alex Chernyakhovsky <achernya@google.com>
Reviewed-by: Paul Wankadia <junyer@google.com>