[docs] post-processing: a look for one camera, and the Set Look node

Two sections, both written from what a user got wrong first time.

A look for one camera: the Look for picker, and the thing that is not obvious —
a camera look only shows WHILE SOMEONE IS LOOKING THROUGH that camera, so
adding effects to one leaves the viewport unchanged until you preview it or a
Set Active Camera node takes you there. That silence was the first thing
reported about this feature, so the page says it in the place you would be
standing when it happens. Plus the Combine table (add vs replace) and the note
that effects still merge across the join.

Switching looks from a node graph: the Set Look node, with 'look through it
too' explained as on-by-default and why (it is what makes the look visible),
the worked example that was actually attempted — R and U on two cameras — and
the two properties worth trusting: the switch is per viewer and temporary, so
it never edits the saved look, and nobody's graph moves anybody else's view.

Author: AlexZ005

docs/post-processing.md

@@ -90,6 +90,58 @@ The look renders for everybody. What is local is your right to switch it off:
 - Turning an effect off in the stack, or unticking **Scene look enabled**, changes it *for
   everyone* — that is scene data, not a personal setting.
 
+## A look for one camera
+
+A look does not have to belong to the whole scene. Any [camera](camera.md) can carry
+its own, so switching cameras switches the grade — a hero camera with a heavy
+cinematic look, a security-monitor camera that is deliberately grey and grainy.
+
+In **Post-processing**, the **Look for** row picks which look you are editing: *The
+scene (everyone)*, or any camera in the scene. Pick a camera and you get a fresh,
+empty list — build it exactly the same way.
+
+A camera look only shows **while someone is looking through that camera**. So after
+you add effects to one, the viewport does not change until you look through it —
+select the camera and press **Preview**, or use a **Set Active Camera** node.
+
+### Add, or replace
+
+The **Combine** row decides how a camera look meets the scene look:
+
+| | |
+|---|---|
+| **Add to the scene look** | The scene look runs first, then this camera's. The usual choice: the house look plus a grade for this shot. |
+| **Replace the scene look** | Only this camera's look runs. For a camera that is deliberately not the house look. |
+
+Effects still merge across the join, so a scene grade plus a camera grade is usually
+still one fullscreen pass.
+
+Camera looks are scene data like everything else here: they replicate, they save with
+the file, and they undo.
+
+## Switching looks from a node graph
+
+The **Set Look** node (Game group) switches a look when something happens — a key
+press, a trigger, a game state.
+
+- **camera** — whose look. Leave it empty to target the scene look. Wire an Object
+  Selector, or pick from the list on the card.
+- **look** — on or off.
+- **look through it too** — on by default, and usually what you want: the node points
+  your view at that camera as well, which is what makes its look visible. Turn it off
+  when you only want to arm or disarm a look without moving anyone.
+
+So *Key Press R → Set Look (camera A)* and *Key Press U → Set Look (camera B)* gives
+you two grades on two keys.
+
+The switch is **per viewer and temporary**: it never edits the saved look, so turning
+one off in a game does not change what anyone has authored. Each peer acts on the
+same replicated trigger, so nobody's graph moves anybody else's view.
+
+If you turn *look through it too* off and nothing is looking through that camera, the
+node has nothing visible to do — the card says so while you build, and it tells you
+once if you fire it anyway.
+
 ## Limits worth knowing
 
 - **Post-processing does not run in VR.** The effects are skipped in a headset; objects