Map: document explicitly that initial regions are immutable.

Chris Boyle reports that a few users of the Android port were confused
by this, e.g. https://github.com/chrisboyle/sgtpuzzles/issues/624 .

(That seems surprising to me, since I view Map as extremely closely
related to Solo - both are special cases of the general game class
'here is a partial k-colouring of a graph, find the unique total
k-colouring that extends it', just with different ranges of k and
different valid graphs. And surely nobody approaches a Sudoku puzzle
and expects to be able to rub out provided clues they don't like! But
I suppose if you're thinking of Map as a completely separate puzzle
then perhaps that analogy doesn't have the same force.)

puzzles.but

@@ -1923,7 +1923,7 @@ You are given a map consisting of a number of regions. Your task is
 to colour each region with one of four colours, in such a way that
 no two regions sharing a boundary have the same colour. You are
 provided with some regions already coloured, sufficient to make the
-remainder of the solution unique.
+remainder of the solution unique, and these cannot be changed.
 
 Only regions which share a length of border are required to be
 different colours. Two regions which meet at only one \e{point}