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.
* Fix a typo
* Fix lots of spelling errors
* Fix a few more spelling mistakes
* s/parsable/parseable/
* Don't touch the third party files
* Cloneable is the preferred C# term
* Copyable is the preferred C++ term
* Revert "s/parsable/parseable/"
This reverts commit 534ecf7675.
* Revert unparseable->unparsable corrections
This includes instructions for enabling explicit presence tracking in proto3, and also explains implicit presence. The language examples were written by inspection, and not individually tested.
