Be completely clear about what g_direct_hash, g_direct_equal do
Be completely clear about what g_direct_hash, g_direct_equal do

Also annotate them as (allow-none), more for the benefit of gtk-doc
readers than introspection.

Bug: https://bugzilla.gnome.org/show_bug.cgi?id=592715
Signed-off-by: Simon McVittie <2b470b34502dbf3ba4bcc04ea7e64f4e0e104f33@collabora.co.uk>
Reviewed-by: Colin Walters <694ce38231ed622bd1593ce4ec9aa85c9ddb0a78@verbum.org>

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