Clarify TpPresenceMixinStatusAvailableFunc
Clarify TpPresenceMixinStatusAvailableFunc

Most CMs don't need to use this function at all. It's only really for
the benefit of Gabble. So let's document up-front that it should usually
be NULL, and explain the situation where it should not be NULL in the
documentation for the typedef.

Distrotech/telepathy-glib,Distrotech/telepathy-glib,Distrotech/telepathy-glib,Distrotech/telepathy-glib,Distrotech/telepathy-glib