Add comment explaining what it means if feature->prepare_async is NULL
Add comment explaining what it means if feature->prepare_async is NULL

https://bugs.freedesktop.org/show_bug.cgi?id=49215

Distrotech/telepathy-glib,Distrotech/telepathy-glib,Distrotech/telepathy-glib,Distrotech/telepathy-glib,Distrotech/telepathy-glib