Document the use of g_autofree and g_autoptr in glib for automatic
freeing of memory.
Backports commit 821f2967562a1fdc7e52a644963163e6917c4293 from qemu
The split of information between the two docs is rather arbitary and
unclear. It is simpler for contributors if all the information is in
one file.
Backports commit 637f39568fc0bd9848fd9d225d52ab0c4c443ed3 from qemu