Yelp platform detection of Unity

[Kevin Godby]

Hello, everyone.

I noticed that yelp (the desktop documentation viewer) wasn't detected
the Unity platform and was instead displaying instructions for GNOME.

I've filed a bug (with a patch) to resolve this issue:
<https://bugzilla.gnome.org/show_bug.cgi?id=707352>

I'm not sure if it will get resolved in time for 13.10 or not.  I just
wanted to give everyone a heads-up.

Thanks!

—Kevin Godby