src: update f function call comment

Commit a01e8bcf (src: moving f function call comment, 2016-09-04) moved
a comment to be closer to the place in the code it was describing.
While working on this there was feedback on the comment
itself:
https://github.com/nodejs/node/commit/
a01e8bcf#comments

This PR includes the suggestions in the above comments.

PR-URL: https://github.com/nodejs/node/pull/8416


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Franziska Hinkelmann <franzih@chromium.org>
Reviewed-By: James M Snell <jasnell@gmail.com>