Emmanuele Bassi authored Dec 13, 2018 and Philip Withnall committed May 24, 2019

As we discovered in gtk#1280, GCC considers the pragmas to control
the deprecation warnings as statements. This means we cannot just use
the GLib wrappers as markers around the call site, but we must be aware
of their side effects.

Let's document this, to avoid falling into the trap.