GtkAboutDialog: Don't shout

Don't use all-caps in the UI - it looks scary.

https://bugzilla.gnome.org/show_bug.cgi?id=671138
Improve tooltip-text/markup docs

Clarify priorities between the two,
https://bugzilla.gnome.org/show_bug.cgi?id=681185
Add GtkGLArea fully to docs

It was missing from the type list.