docs: There is no decodebin2 anymore, don't pretend otherwise
docs: Cleanup interface references in docs

 * Delete references to removed interfaces
 * Add missing documentation sections
 * Fix duplicate interface references for GstDevice

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=732786
docs: Fix documentation typos and inconsistencies

 * GstGlobalDeviceMonitor was renamed to GstDeviceMonitor
 * Expand GST_MESSAGE_DEVICE to the full enum value names
 * Correct the incorrect references to the GstDeviceProvider interfaces
 * Describe caps arguments for gstcheck interface
 * Add missing docs for GstNetAddressMeta and its add function
 * Add docs for toc helper macros
 * Avoid refering to GstValueList type as done elsewhere

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=732786