Updates to GIO documentation. (#506395, Mikael Hermansson)

* 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

gio/gdrive.c

@@ -33,6 +33,10 @@
  * @short_description: Virtual File System drive management
  * @include: gio.h
  * 
+ * #GDrive - this represent a piece of hardware connected to the machine.
+ * Its generally only created for removable hardware or hardware with
+ * removable media. 
+ *
  * #GDrive is a container class for #GVolume objects that stem from
  * the same piece of media. As such, #GDrive abstracts a drive with
  * (or without) removable media and provides operations for querying