doc: fix a few n-api doc issues

• Add doc for napi_create_string_latin1().
• Fix signatures where c string was specified instead of napi_value.
• Fix return type of napi_callback.
• Update to specify that napi_escape_handle() can only be called once for a given scope.

Fixes: https://github.com/nodejs/node/issues/13555 Fixes: https://github.com/nodejs/node/issues/13556 Fixes: https://github.com/nodejs/node/issues/13562

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• documentation is changed or added
• commit message follows [commit guidelines]

Affected core subsystem(s)

doc, n-api