Fix ambiguous comment
Fix ambiguous comment

I intended "arbitrary non-NULL" to mean "only the fact that it is not
NULL is API", but sjoerd briefly thought I meant "user-defined non-NULL".

freedesktop-unofficial-mirror/telepathy__telepathy-mission-control,freedesktop-unofficial-mirror/telepathy__telepathy-mission-control,freedesktop-unofficial-mirror/telepathy__telepathy-mission-control,freedesktop-unofficial-mirror/telepathy__telepathy-mission-control