It seems the Mint team did this intentionally because "yelp" doesn't contain Cinnamon help, but rather Gnome-Shell help (or in Ubuntu's case, Unity help). But I personally think that the help is very essential for application help, so I reverted this hack by the Mint team. Here is how to do it:
sudo mv /usr/local/bin/yelp /usr/local/bin/yelp.mint
(It is this file that I think the Mint team made: since usr/local/bin comes before /usr/bin in the PATH, it is called first, effectively bypassing the actual yelp program. By renaming this file, the "real" yelp is called in /usr/bin/yelp instead of this hacked shortcut to linuxmint help online).
Here is the reference from Clem's response to a comment confirming that MInt did this intentionally:http://blog.linuxmint.com/?p=2010
Search for "yelp" on that page and and you will find the discussion.