GActionEntry: improve documentation

Document that giving %NULL for the activate handler is supported since
GLib 2.40.  We documented this on GSimpleAction itself (where the
default handler functionality is implemented) but expecting the user to
dig that up is asking a bit much.

Also, add some more explicit documentation about the conditions under
which each field is expected to be filled in.

https://bugzilla.gnome.org/show_bug.cgi?id=732429