Fixes and lua doc

doc/fixes37.0

@@ -588,6 +588,7 @@ redo rndmonst() to operate in a single pass (github pull request #286)
 fix the "stuck pets" issue (github #329)
 allow themed room subrooms to be filled (github #347)
 allow rereading spellbooks to refresh memory at any time (github #261)
+allow themed rooms constrained by level difficulty (github #344)
 
 
 Code Cleanup and Reorganization

doc/lua.adoc

@@ -111,6 +111,15 @@ Example:
  local str = nh.ing_suffix("foo");
 
 
+=== level_difficulty
+
+Returns an integer value describing the level difficulty.
+Normally this is the level's physical depth from the surface.
+
+Example:
+
+ local diff = nh.level_difficulty();
+
 === makeplural
 
 Pluralize the given string.