gmain: More explicitly document g_main_context_release() prereqs
The documentation was fairly clear before, but we can make it clearer:
it’s a programming error to call g_main_context_release()
if you have
not received a true return value from g_main_context_acquire()
before.
Inspired by !3302 (closed).
Signed-off-by: Philip Withnall pwithnall@endlessos.org