Add more information about how get_node_light works.

doc/lua_api.txt

@@ -1914,9 +1914,13 @@ and `minetest.auth_reload` call the authetification handler.
     * Returns `{name="ignore", ...}` for unloaded area
 * `minetest.get_node_or_nil(pos)`
     * Returns `nil` for unloaded area
-* `minetest.get_node_light(pos, timeofday)` returns a number between `0` and `15` or `nil`
+* `minetest.get_node_light(pos, timeofday)`
+    * Gets the light value at the given position. Note that the light value
+      "inside" the node at the given position is returned, so you usually want
+      to get the light value of a neighbor.
+    * `pos`: The position where to measure the light.
     * `timeofday`: `nil` for current time, `0` for night, `0.5` for day
-
+    * Returns a number between `0` and `15` or `nil`
 * `minetest.place_node(pos, node)`
     * Place node with the same effects that a player would cause
 * `minetest.dig_node(pos)`