Describe Tone Mapping and the film's new place in the manual

The Light group gains `Tone Mapping`, and a film stock now takes its
place at the end of the chain rather than sitting after exposure (D19).
Say what the two sliders do and why every adjustment above them works on
the scene, highlights beyond white included. The pictures still show the
old rendering and are re-recorded separately.

docs/manual/README.md

@@ -174,6 +174,15 @@ the strip at its head narrows it to one group.
 Exposure, contrast, highlights, shadows, blacks, whites and a tone curve.
 Hold `Before` to see the photograph as it was.
 
+`Tone Mapping`, last in the group, is how the scene is fitted onto the
+screen, and it runs after every other adjustment. `White Point` says how many
+stops above middle grey reach white — raise it to bring a bright sky back
+from white, lower it for a brighter, punchier picture — and `Contrast` sets
+the slope of the curve between. Every adjustment above it works on the scene
+as the camera recorded it, highlights beyond white included, so pulling the
+highlights recovers what the sensor caught rather than what the screen could
+show.
+
 ![Raising exposure, pulling the highlights, lifting the shadows, then holding Before](media/develop-light.gif)
 
 ### Looking closer
@@ -269,7 +278,9 @@ opacity are in the panel. Heal blends; clone copies.
 
 The `Film` chooser at the head of Adjust applies a spectral simulation of a
 named stock; below it, the print exposure and push controls a film has and a
-sensor does not. The list opens over the column and scrolls on its own — by
+sensor does not. A stock takes the place of `Tone Mapping`: it is the last
+thing that happens to the picture, so every other adjustment decides the
+exposure the negative receives. The list opens over the column and scrolls on its own — by
 wheel, drag or flick, or with `Up`, `Down` and `Enter` — down to the
 black-and-white stocks at its end.