Messages: clarify PendingMessages docs
Messages: clarify PendingMessages docs

Calling this list a superset implies that it might have more messages,
which is false. It might have *more information*, though, so I'll say
"more detailed".

Also add <tp:dbus-ref> to replace clumsy wording with hyperlinks.

(As per review from wjt)

TelepathyIM/telepathy-spec,freedesktop-unofficial-mirror/telepathy__telepathy-spec