Gio::content_type_*(): Improve documentation

* gio/giomm/contenttype.h: Add @defgroup giommContentType. Useful as
a link target from Gtk::AppChooser, similar to a link from
GtkAppChooser:content-type to GContentType. See gtk+ bug #650012.
Several minor fixes in content_type_*() docs.
2.38.1
This test assumes that the first DNS result it gets for www.google.com is an
IPv4 address -- but it can equally well be an IPv6 address. Create the right
kind of socket to match.
Merge branch 'master' of git://git.gnome.org/glibmm
JOHNE - Bump the version number to 2.38.01