Add Settings interface for Lua

doc/menu_lua_api.txt

@@ -182,3 +182,9 @@ string:split(separator)
 ^ eg. string:split("a,b", ",") == {"a","b"}
 string:trim()
 ^ eg. string.trim("\n \t\tfoo bar\t ") == "foo bar"
+minetest.is_yes(string)
+^ returns whether string can be interpreted as yes
+
+Class reference
+----------------
+Settings: see lua_api.txt