sys/luid: Added postcondition to luid_get()

Stated the obvious in the doc: Altering the LUID returned by luid_get() breaks
the guarantee that it is locally unique, as it than could collide with other
LUIDs returned by luid_get().

sys/include/luid.h

@@ -75,6 +75,8 @@ extern "C" {
  * byte.
  *
  * @note    The resulting LUID will repeat after 255 calls.
+ * @post    The generated LUID must be used unchanged, as it could collide with
+ *          other LUIDs if altered.
  *
  * @param[out] buf      memory location to copy the LUID into. MUST be able to
  *                      hold at least @p len bytes