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.

data/core/help.cfg

@@ -494,11 +494,41 @@ Thick-bodied and clumsy, slow individuals of goblins and other species take a mo
         title= _ "Terrains"
         text= _ "Game maps feature a variety of terrains that affect both unit movement and a unit’s defensive capability in combat." + _"
 
-Terrains come in two types: <italic>text='basic'</italic> terrain types and <italic>text='mixed'</italic> terrain types. Descriptions of the basic terrain types are listed in this section. Mixed terrain types share properties of multiple basic terrain types -- units generally recieve the <italic>text='best defense'</italic> and <italic>text='worst movement'</italic> of the underlying basic types when they move onto a mixed type. For example, this is the case with <italic>text='forested hills'</italic>, <italic>text='sand hills'</italic>, and <italic>text='cave hills'</italic>. One notable exception is <italic>text='bridge'</italic>-type terrains, such as bridges over shallow water, fords, and bridges over chasms. Fords are easily passable to both merfolk and humans -- all units moving on a ford enjoy the best defense and best movement out of flat and shallow water, rather than the worse movement of the two. Similarly, bridges over chasms are passable to nonfliers (unsurprisingly)." + _"
+Terrains come in two types: <italic>text='basic'</italic> and <italic>text='mixed'</italic>." + _"
 
-You can see what basic types are associated to a mixed type by mousing over its hex and checking the terrain type icons displayed in the upper right, under the default theme." + _"
+<header>text='Basic Terrain Types'</header>" + _"
 
-Some units have <italic>text='defense caps'</italic> for a particular basic terrain type. In that case, the 'best defense' behavior is overridden -- they can never recieve a higher defense rating than the lowest defense cap value for any mixed terrain type which includes that basic type. For example, the loyalist cavalryman has a defense rating of 30% on forests, and a defense cap for forests. Thus, on forested hills, he has a defense rating of 30% rather than 40%, because the mixed rating cannot exceed the cap." + _"
+Basic terrain types include Flat, Hills, Mountains, Sands, Water and Swamps. There are many more besides these — a list of the terrain types you have discovered can be found at the end of this page.
+
+" + "<img>src='terrain/grass/green.png'</img>" + "<img>src='terrain/hills/regular.png'</img>" + "<img>src='terrain/mountains/dry-tile.png'</img>" + "<img>src='terrain/sand/desert.png'</img>" + "<img>src='terrain/water/coast-tile.png'</img>" + "<img>src='terrain/swamp/water-tile.png'</img>" + _"
+
+Every unit has a defense rating and a movement cost for each of the basic terrain types, and these values are listed in the help page of the unit. Basic terrain types may have unique properties like illumation effects as well." + _"
+
+<header>text='Mixed Terrain Types'</header>" + _"
+
+Mixed terrain types share the properties of multiple basic terrain types — units generally recieve the <italic>text='best defense'</italic> and <italic>text='worst movement'</italic> of the underlying basic types when they move onto a mixed type. For example, this is the case with <italic>text='forested hills'</italic>, <italic>text='sand hills'</italic>, and <italic>text='cave hills'</italic>.
+
+" + "<img>src='terrain/hills/regular.png~BLIT(terrain/forest/pine-tile.png)'</img>" + "<img>src='terrain/hills/desert.png'</img>" + "<img>src='terrain/cave/hills-variation.png'</img>" + _"
+
+One notable exception is bridge terrains, such as <italic>text='bridges over shallow water'</italic>, <italic>text='fords'</italic>, and <italic>text='bridges over chasms'</italic>. Fords are easily passable to both merfolk and humans — all units moving on a ford enjoy the best defense and best movement out of flat and shallow water, rather than the worse movement of the two. Similarly, bridges over chasms are passable to nonfliers (unsurprisingly).
+
+" + "<img>src='terrain/water/coast-tile.png~BLIT(terrain/bridge/wood-se-nw.png)'</img>" + "<img>src='terrain/water/ford-tile.png'</img>" + "<img>src='terrain/chasm/regular-tile.png~BLIT(terrain/cave/chasm-stone-bridge-sw-ne-tile.png)'</img>" + _"
+
+Land-based villages generally give best-defense, best-movement as well. These villages are mixed terrains, based on the village terrain type, together with hill, swamp, and cave, respectively.
+
+" + "<img>src='terrain/hills/regular.png~BLIT(terrain/village/human-hills-tile.png)'</img>" + "<img>src='terrain/swamp/water-tile.png~BLIT(terrain/village/swampwater-tile.png)'</img>" + "<img>src='terrain/cave/floor6.png~BLIT(terrain/village/cave-tile.png)'</img>" + _"
+
+Finally, water villages generally are not hospitable to land units, and do not give defense or movement benefits associated to the village terrain type. They count only as water tiles instead.
+
+" + "<img>src='terrain/water/coast-tile.png~BLIT(terrain/village/coast-tile.png)'</img>" + _"
+
+You can see what basic types are associated to a mixed type by mousing over its hex and checking the terrain type icons displayed in the upper right, (under the default theme)." + _"
+
+<header>text='Defense Caps'</header>" + _"
+
+Some units have <italic>text='defense caps'</italic> for a particular basic terrain type. These units suffer a penalty on mixed terrains with that type — their defense can't exceed any of the caps." + _"
+
+For example, the <ref>text='Loyalist Cavalryman' dst='..unit_Cavalryman'</ref> has a defense rating of 30% on forests, and a defense cap for forests. Thus, on forested hills, he has a defense rating of 30% rather than 40%, because the mixed rating cannot exceed the cap." + _"
 
 If a unit has a defense cap for some terrain, it is always the same as its defense rating on that terrain (it cannot be larger)." + _"