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Devel docs: explain the specialness of colour 0

It's used to fill the drawing area and also by some front-ends (at
least GTK and JavaScript) to fill areas around the drawing area.

--- a/devel.but

+++ b/devel.but

@@ -33,7 +33,7 @@

 new platform.

 This guide is believed correct as of \cw{git} commit

-\cw{b967a3ea86d6285457abc2a89e455c8f62d2c22a}. Hopefully it will be

+\cw{9afdd4cca4823fa3b27ad2ca2fec33e04d7ab103}. Hopefully it will be

 updated along with the code in future, but if not, I've at least left

 this version number in here so you can figure out what's changed by

 tracking commit comments from there onwards.

@@ -1220,6 +1220,12 @@

 end's default colour as their background, apart from a few which

 depend on drawing relief highlights so they adjust the background

 colour if it's too light for highlights to show up against it.

+

+The first colour in the list is slightly special. The mid-end fills

+the drawing area with it before the first call to \cw{redraw()} (see

+\k{backend-redraw}).  Some front ends also use it fill the part of the

+puzzle window outside the puzzle.  This means that it is usually

+sensible to make colour 0 the background colour for the puzzle.

 Note that the colours returned from this function are for

 \e{drawing}, not for printing. Printing has an entirely different