docs: Update the documentation for G_GNUC_MALLOC to reflect recent GCC

docs: Update the documentation for G_GNUC_MALLOC to reflect recent GCC

Thanks to some great investigation by Benjamin Moody, it’s clear that
our documentation and usage of G_GNUC_MALLOC has fallen behind GCC’s
interpretation of the malloc attribute, meaning that recent versions of
GCC could miscompile code which uses G_GNUC_MALLOC incorrectly.

Update the documentation of G_GNUC_MALLOC to match the current GCC
documentation (for GCC 8.2). Following commits will drop our use of
G_GNUC_MALLOC from inappropriate functions.

Specifically, the change in GCC’s interpretation of the malloc attribute
which could cause miscompilation is that returned storage areas are now
assumed to not contain valid pointers — so realloc() cannot have the
malloc attribute, and neither can a function which returns a newly
allocated structure with fields initialised to other pointers.

Signed-off-by: Philip Withnall <withnall@endlessm.com>
https://gitlab.gnome.org/GNOME/glib/issues/1465