GObject Introspection and/or gi-docgen automatically add getter/setter-to-property cross references, so there's no need to annotate them manually any more.
The PurpleAddContactRequest docs seems a bit copy/paste from PurpleAuthorizationRequest, so I tweaked them a bit and it's hopefully a bit clearer.
Also, fix a couple typos in function names.

Built docs and checked properties still had some reference to the getter/setter, and vice versa.