in the debug mode terrain descriptions, also give the editor image

This is helpful if you are trying to figure out what to put in the
<img> tags when inserting an image for a terrain in a help entry.
add an explanation of mixed terrain types to the help for terrains

This commit fleshes out the help page for terrains. It explains
mixed terrains and the different kinds of behavior they exhibit,
using images to give examples. These images use the same links
used on the terrain type help images.

This help page could be further improved by adding links to
terrain type descriptions in text, and also by fixing the link to
the page about Loyalist Cavalryman which I didn't get to work.
Merge branch 'master' of git://github.com/wesnoth/wesnoth
update changelog