Galaxies: clarify wording of completion condition.

Galaxies: clarify wording of completion condition.

A user mailed me today having found it less than clear from the docs
that Galaxies will only accept a solution if the set of filled-in grid
edges consists of _exactly_ the ones that separate two distinct
regions, rather than consisting of _at least_ those and perhaps others
which neither break rotational symmetry or disconnect any region.