Update docs for AddRequestLobbyListFilterSlotsAvailable and GetNumLobbyMembers

[apkatsikas]

These docs were inaccurate. GetNumLobbyMembers can return the number of users without being a member of the Lobby, as shown in the official docs - https://partner.steamgames.com/doc/api/ISteamMatchmaking

Also, through testing I found that AddRequestLobbyListFilterSlotsAvailable will show rooms with more than the specified number available, so it is an "at least" filter.

[rlabrecque]

This one's going to be hard, as those docs are automatically pulled from Valve's SDK and this change would be overwritten upon the next generate