Lua_api.txt: Add note of maximum value for node 'light_source'

Maximum is 14.
A value of 15 (reserved for direct sunlight) causes a lighting bug.

doc/lua_api.txt

@@ -3664,7 +3664,7 @@ Definition tables
         ^ Don't forget to use "leveled" type nodebox. ]]
         liquid_range = 8, -- number of flowing nodes around source (max. 8)
         drowning = 0, -- Player will take this amount of damage if no bubbles are left
-        light_source = 0, -- Amount of light emitted by node
+        light_source = 0, -- Amount of light emitted by node (max. 14)
         damage_per_second = 0, -- If player is inside node, this damage is caused
         node_box = {type="regular"}, -- See "Node boxes"
         connects_to = nodenames, --[[