gmem: clarify that g_malloc always uses the system allocator

Support for custom allocators was dropped in

commit 3be6ed60 Author: Alexander Larsson alexl@redhat.com Date: Sat Jun 27 18:38:42 2015 +0200

Deprecate and drop support for memory vtables

The introductory doc text for the gmem APIs still warns against mixing malloc/free with g_malloc/g_free. Clarify upfront in the docs that these two sets of APIs are now guaranteed to use the same memory allocator & can thus their usage can be freely mixed.

Signed-off-by: Daniel P. Berrangé berrange@redhat.com