add comment about plugin docs

library/LuaApi.cpp

@@ -3133,7 +3133,10 @@ static int internal_getCommandHelp(lua_State *L)
             {
                 help += ".";
             }
-            help += "\n" + pc.usage;
+            if (pc.usage.size())
+            {
+                help += "\n" + pc.usage;
+            }
             lua_pushstring(L, help.c_str());
             return 1;
         }

library/include/PluginManager.h

@@ -97,6 +97,10 @@ namespace DFHack
         /// create a command with a name, description, function pointer to its code
         /// and saying if it needs an interactive terminal
         /// Most commands shouldn't require an interactive terminal!
+        /// Note that the description and usage fields are only used for
+        /// out-of-tree plugins that do not have rendered help installed in
+        /// the hack/docs directory. Help for all internal plugins comes from
+        /// the rendered .rst files.
         PluginCommand(const char * _name,
                       const char * _description,
                       command_function function_,