Actually document what minetest.is_protected should do

doc/lua_api.txt

@@ -2300,6 +2300,9 @@ These functions return the leftover itemstack.
     * currently supported.
     * `...` indicates method-specific arguments. Currently, no methods use this.
 * `minetest.is_protected(pos, name)`: returns boolean
+    * Returns true, if player `name` shouldn't be abled to dig at `pos` or do other
+      actions, defineable by mods, due to some mod-defined ownership-like concept.
+      Returns false or nil, if the player is allowed to do such actions.
     * This function should be overridden by protection mods and should be used to
       check if a player can interact at a position.
     * This function should call the old version of itself if the position is not