docs: Split GSocketConnectable/GProxyAddressEnumerator documentation

Putting them in the same section causes gtk-doc to mix their properties
together in a single listing, which is confusing.

Since having them in a single section doesn’t really add anything, split
them out to one class per section.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

https://bugzilla.gnome.org/show_bug.cgi?id=791334