lib/commit: Add some gtk-doc and internal doc comments
lib/commit: Add some gtk-doc and internal doc comments

Just making more of an effort for this for obvious reasons. We had a few public
APIs not documented too.

Closes: #1230
Approved by: jlebon

GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree