De-couple from original code

* `api.lua` create separate handler for listing registered nodes, leave the areas list clean
* `hud.lua` call second handler separately
* `api.md` extended and clarified documentaion

api.md

@@ -5,13 +5,38 @@ Adding your protections to the HUD
 
 If you are providing an extra protection mod to work in cunjunction with the
 HUD feature of `areas`, you can register a callback to add your mod's code to
-display your protection's existence. For example
+display your protection's existence.
 
-	local myhandler = function(pos,arealist)
+Registering a handler:
+
+* `areas.register_hud_handler(handler_name) --> nil`
+
+Handler specification:
+
+* `handler_name(pos,area_list) --> new_area_list`
+	* `pos` - the position at which to check for protection coverage by your mod
+	* `area_list` - the current list of protected areas
+	* `new_area_list` - the list of protected areas, updated with your entries
+
+Area list items:
+
+The area list item is a map table identified with an ID, and properties
+
+The ID should be in the format `modname:` and appended with an identifier for the protection.
+
+Each area list item should be a table with the following properties
+
+* `owner` - (required) the name of the protection owner
+* `name` - (optional) the name of the area
+
+Example
+-------
+
+	local myhandler = function(pos,area_list)
 		local areaowner = find_my_protections(pos)
 
 		if areaowner then
-			arealist["mymodname"] = {
+			arealist["mymodname:first"] = {
 				name = "Protection name",
 				owner = areaowner, 
 			}