Add stub documentation for McpRequest
Add stub documentation for McpRequest

I'm not documenting this class until the smcv/requests branch lands, since
its API gets more capable in that branch.

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