Update Field Presence doc

[VeryStrongFingers]

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.

Now the doc infers the feature is implicitly enabled, rather than an experimental flag which was only true up until v3.15.0

see notes on optional/--experimental_allow_proto3_optional here:

• https://github.com/protocolbuffers/protobuf/releases/tag/v3.15.0
• https://github.com/protocolbuffers/protobuf/releases/tag/v3.12.0

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[VeryStrongFingers]

@googlebot I signed it!

[haberman]

I think this should be "use"?

[VeryStrongFingers]

Good catch. Fixed that up