doc: clarify symlink resolution for __filename

Make it more explicit that symlinks are resolved in `__filename`.

Refs: https://github.com/nodejs/node/issues/22602#issuecomment-440906602
PR-URL: https://github.com/nodejs/node/pull/24587


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>