[Fedora-packaging] guideline for OnlyShowIn?

[Rex Dieter]

Per:
"Base KDE applications (e.g. ksysguard) should have OnlyShowIn=KDE"
https://bugzilla.redhat.com/show_bug.cgi?id=481739

Prompted me to try to come up with a good/simple guideline when it is and is
not appropriate to use OnlyShowIn.  Problem is, I couldn't, not easily or
quickly anyway.

I thought I'd start on the packaging list first, any thoughts?

-- Rex