doc: add missing n-api version indicator

When the threadsafe functions were promoted to
version 4 from experimental we missed documenting
that. Add the version indicator for these functions
in the doc.

PR-URL: https://github.com/nodejs/node/pull/27155


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>