Show the picker correcting something in the manual

The recording sampled a red brick wall and moved the sliders by three
units, which at GIF size is a click that does nothing. The scene now
drags the frame cold first and picks a white air conditioner, so the
correction is visible and the picker's being absolute - set from the
photograph, not from where the sliders were - is what the picture
shows. The text says so, and says a blown highlight is refused.

docs/manual/README.md

@@ -99,9 +99,13 @@ wheel zooms to any amount in between.
 
 ### White balance from the photograph
 
-Press `pick` in the White Balance group, then click something neutral.
+Press `pick` in the White Balance group, then click something neutral —
+a white wall, a grey card, the air conditioner here. The picker sets the
+sliders from the photograph, not from where they were: below, the frame
+is dragged cold first and one click puts it right. A blown highlight is
+refused, since a clipped pixel has no colour left to balance.
 
-![Picking a neutral wall to set the white balance](media/develop-wb.gif)
+![Cooling the frame with the slider, then picking a white air conditioner to set the white balance](media/develop-wb.gif)
 
 ### Composing
 

tools/manual/scenes.py

@@ -207,8 +207,9 @@ def develop_wb():
     group('colour')
     rec('develop-wb')
     pause(0.4)
+    dr.drag(1398, 826, 1330, 826, 25); pause(1.2)      # temperature cold
     dr.click(1487, 776); pause(0.8)                    # pick
-    dr.click(1000, 300); pause(1.5)                    # a neutral wall
+    dr.click(995, 635); pause(1.5)                     # the white air conditioner
     hold(*BEFORE, 1.4); pause(0.8)
     cut()
     reset_edit()