Update Field Presence doc
Update Field Presence doc

Updated information surrounding the usage of `optional` and the `--experimental_allow_proto3_optional` flag with relevant information between v3.12 & 3.15.

Since release v3.15:
>  Optional fields for proto3 are enabled by default, and no longer require
>  the --experimental_allow_proto3_optional flag.
google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf