core: Document WritableContainer some more
core: Document WritableContainer some more

Provide more information on what exactly is expected and not expected from
WritableContainer implementations.

GNOME/rygel,GNOME/rygel,luisbg/rygel,jku/rygel,Tarnyko/rygel,Pelagicore/Media-Manager-Rygel-patched,Pelagicore/Media-Manager-Rygel-patched,jku/rygel,Pelagicore/Media-Manager-Rygel-patched,Tarnyko/rygel,GNOME/rygel