Add comment about changing terrains.

When changing terrains for villages they will disown teams when needed,
but never capture for a team. This commit explains why this hasn't been
done. (The information is also on the wiki.)

git-svn-id: http://svn.gna.org/svn/wesnoth/trunk@56266 75b3775d-3801-0410-9e12-e2e43121521d