docs: Include references to ptz capability for substreams. (#2120)

- Closes: #2118

docs/configuration/cameras/README.md

@@ -67,6 +67,10 @@ cameras:
 | `substream`             | The camera can be used as a substream on another camera.                                          |
 | `trigger`               | The camera can be triggered.                                                                      |
 
+?> If using a camera only as a `substream`, don't forget to keep both the
+`substream` and `ptz` capabilities enabled if you wish to use PTZ controls for
+the substream.
+
 ## `cast`
 
 The `cast` block configures how a camera is cast / sent to media players.

docs/examples.md

@@ -58,6 +58,8 @@ cameras:
     capabilities:
       disable_except:
         - substream
+        # Also allow PTZ controls on the substream.
+        - ptz
 automations:
   - conditions:
       - condition: fullscreen
@@ -504,6 +506,8 @@ cameras:
     capabilities:
       disable_except:
         - substream
+        # Also allow PTZ controls on the substream.
+        - ptz
 automations:
   - actions:
       - action: custom:advanced-camera-card-action
@@ -1087,10 +1091,12 @@ cameras:
     title: Office HD
     live_provider: go2rtc
     id: office_hd
-    # This camera serves only as a substream.
     capabilities:
       disable_except:
+        # This camera serves only as a substream.
         - substream
+        # Also allow PTZ controls on the substream.
+        - ptz
 menu:
   buttons:
     substreams: