Clarify documentation for g_log_set_debug_enabled()

The following discussion from !2515 (merged) should be addressed:

• @mwleeds started a discussion: (+2 comments)

  I think it would also be good to improve these docs to say whether the debug output being enabled is for all log domains or a particular one. There's a mention of it being "domain-dependent" in the docs for g_log_get_debug_enabled(), but it's still not clear to me.