docs(tools/matrix): add invite_user, join_room, list_invites to matrix tool docs

docs/tools/matrix.md

@@ -23,6 +23,16 @@ a second stdio MCP server. Tools land as `mcp__matrix__<name>`:
   (`{ id, canonical_alias, name, member_count }` per room)
 - `list_room_members(room)` — members of a room
 
+### Room membership
+
+- `invite_user(room, user_id)` — invite `@user:server` into a room
+  you're already in; you must have a high enough power level.
+  The invitee sees a pending invite and accepts via `join_room`.
+- `join_room(room)` — join a room by id or alias, or accept a
+  pending invite.
+- `list_invites()` — rooms this agent has been invited to but not
+  yet joined (`{ id, canonical_alias, name }` per room).
+
 ### Receipts
 
 - `mark_read(room, event_id)` — advance the read receipt