Show AI denoise in the manual, on a night frame with no faces

A section after Looking closer: what it is for, the switch and its wait,
Keep grain, which cameras it takes and where its noise figures come from.
The scene opens the Brooklyn Bridge at ISO 8000 from the face-free demo
set at 1:1, switches it on, waits for the result to land and keeps some
grain, with a still before and after. It waits on the app's own log line
rather than a fixed time: the network takes seconds on a GPU and more on
the CPU, which is where the recording X server leaves it (13 s).

docs/manual/README.md

@@ -205,6 +205,35 @@ the sensor recorded.
 
 ![Zooming to 1:1 with a double-click, panning, then further in with the wheel](media/develop-zoom.gif)
 
+### AI denoise
+
+For a photograph taken in poor light at a high ISO. `AI Denoise`, in the
+Detail group, replaces how the camera's raw data is turned into colour: a
+network trained on this library's own photographs removes the noise and
+the blotches of colour that come with it, while keeping the fine detail.
+Look at it at 1:1, where noise lives.
+
+Switch it on with `Apply`. The photograph keeps showing as it was while
+the network works, with its progress in the bar at the top, and changes
+when it is done — a few seconds on a computer with a graphics card, about
+fifteen on its processor alone, longer on the tablet. `Keep grain` puts
+back some of what was removed, as grain without colour, for a picture that
+does not look too smooth.
+
+![An ISO 8000 night frame at 1:1, AI Denoise switched on, then some grain kept](media/develop-denoise.gif)
+
+| Before | After |
+|---|---|
+| ![The railing and the lamp at ISO 8000, as the camera recorded them](media/develop-denoise-before.png) | ![The same, with AI Denoise](media/develop-denoise-after.png) |
+
+It works on raw files from any camera with the usual colour pattern of
+red, green and blue squares — not on JPEGs, and not yet on Fujifilm's
+X-Trans. How noisy the camera is at each ISO was measured for the Canon
+EOS 6D; for other cameras it is read from a DNG's own figures or
+estimated from the photograph, and the finished job in the activity list
+says which. An export uses
+it whenever the photograph has it switched on.
+
 ### Moving between photographs
 
 The roll along the foot of the canvas holds the photographs the grid was