fix a typo s/overrided/overridden/ in help for terrains
Instead of 'yes' show the defense cap value in help.

Suggested by Soliton. Also add a line of explanation in the help
that although a value is shown, if there is a cap it is always
the same as the defensive rating of the unit on that terrain and
cannot be larger.
Merge branch 'master' of git://github.com/wesnoth/wesnoth