Added help text for stickying posts in linkcommentssettings.html

This change adds a help text visible to moderators for "sticky this post" in the same way that contest mode has. It does not appear for users who don't have the ability to sticky the post in the first place, of course. :) All for the sake of congruency and on-page documentation!

![sticky-help](https://f.cloud.github.com/assets/4652603/1030950/3892f5d0-0ecb-11e3-9002-a953af364dfe.jpg)