Help: explicitly sort Terrain sections

[ci skip]

This happened incidentally with the current help implementation, but that no longer happens
with my in-progress refactor. We still want these sections sorted, though.

The removal of the "sort_sections=generated" key from the Eras section is a forward-port of
6b20ccd from 1.14.

data/core/help.cfg

@@ -61,7 +61,6 @@
         id=eras_section
         title= _ "Eras"
         sections_generator=eras
-        sort_sections=generated
         sort_topics=yes
         topics=..eras_section
     [/section]
@@ -72,6 +71,7 @@
         sections_generator=terrains
         topics=..terrains_section
         sort_topics=generated
+        sort_sections=yes
     [/section]
 
     [section]