[Plugin: W3 Total Cache] don't cache comment form

I want to serve a cache to logged in users, except the comment form. W3 TC has fragment caching, but I don’t understand the syntax.

The plugin says:

Example 1:
<!– mfunc any PHP code –><!– /mfunc –>
Example 2:
<!– mfunc –>any PHP code<!– /mfunc –>

so given this call
<?php comments_template(); ?>

Where should I put the mfunc tags?