connector: wanted(port) says whether anyone watches or reads a port

docs/reference/connector-contract.md

@@ -144,6 +144,20 @@ new frames arrive. Use durable storage for recordings.
 Local screens receive pushed live frames; portal screens fetch the bytes.
 Choose a modest poll rate for remote viewing.
 
+`self.wanted(port)` says whether anyone wants what a port publishes right
+now. That is a dashboard tile on screen, a portal screen, or a node of a
+running flow reading its message. Check it before fetching a frame so a camera
+sends nothing while nobody watches:
+
+```python
+async def poll(self):
+    if not self.wanted("frame"):
+        return None
+    ...
+```
+
+On a remote worker it is always true, since the worker cannot see who watches.
+
 ## Keeping state across restarts
 
 Keep connections and device handles in node attributes. Store short strings