* gcontenttype.c: Describe memory management for return value of
g_content_types_get_registered(). Missing piece from #505815.
* gdrive.c, gmount.c, gvolumemonitor.c: Add more description to
GVolume, GDrive, GMounts, which hopefully gives the user less
confusions when using this API. Following explainations from
Alexander Larsson on gtk-devel-list.

svn path=/trunk/; revision=6222