doc: Update supported options

Document the "multipage" option added in commit
09b3fc03284ac4f9a4f466ba908f9cd7309b2ced
lib: Add gdk_pixbuf_get_options() helper

So people don't have to guess the possible values of "key" in
gdk_pixbuf_get_option() calls

https://bugzilla.gnome.org/show_bug.cgi?id=674858