doc: document fs.realpath.native()

Mea culpa, somehow I managed to drop the documentation commit while
merging the pull request.  This should have been included in commit
74023c07 ("fs: expose realpath(3) bindings") from this month.

PR-URL: https://github.com/nodejs/node/pull/17059
Refs: https://github.com/nodejs/node/pull/15776


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>