napi_get_value_string_latin1 not mentioned in N-API docs

[RReverser]

napi_get_value_string_latin1 was introduced in the implementation, but doesn't appear on the documentation page. cc @nodejs/n-api (relevant to #13556)

[mhdawson]

Thanks, I'll take a look and add.

[kfarnung]

@RReverser @mhdawson I have a PR #14678 out for this issue.