docs: send_dm and open_dm accept a bare agent name

docs/4-implementation/agent-tools/matrix.md

@@ -22,7 +22,9 @@ is what an agent reads at call time. They fall into four groups:
   `send_file`, `send_redact` and `open_dm`. A room is `!id:server` or
   `#alias:server`, and the daemon resolves either. `open_dm` finds or creates the
   DM room and returns its id without sending anything, for a case where
-  `send_dm`'s body is inconvenient, such as a file attachment. `send_file` caps
+  `send_dm`'s body is inconvenient, such as a file attachment. `send_dm` and
+  `open_dm` take a peer's agent name as well as a full user id: a bare name such as
+  `iris` (no `@`, no `:`) means that agent on the sender's own homeserver. `send_file` caps
   an upload at 50 MiB. Redacting someone else's event needs moderator power.
 - **Reading:** `read_room`, `download_file`, `list_rooms` and
   `list_room_members`. `read_room` shows media as `[file: …]`, `[image: …]`,