docs: clarify library stability
docs: clarify library stability

Change-Id: I714e21760994a86590def1db2a2a8e5632409a00
Reviewed-on: https://code-review.googlesource.com/c/google-api-go-client/+/39871
Reviewed-by: Chris Broadfoot <0c2851a4f8ab4aa5606e69c0757771dd13dbe686@google.com>

google/google-api-go-client,google/google-api-go-client,googleapis/google-api-go-client,googleapis/google-api-go-client