tal: document tal_steal(ctx, NULL) as OK.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

ccan/tal/tal.h

@@ -131,10 +131,11 @@ void *tal_free(const tal_t *p);
 /**
  * tal_steal - change the parent of a tal-allocated pointer.
  * @ctx: The new parent.
- * @ptr: The tal allocated object to move.
+ * @ptr: The tal allocated object to move, or NULL.
  *
  * This may need to perform an allocation, in which case it may fail; thus
- * it can return NULL, otherwise returns @ptr.
+ * it can return NULL, otherwise returns @ptr.  If @ptr is NULL, this function does
+ * nothing.
  */
 #if HAVE_STATEMENT_EXPR
 /* Weird macro avoids gcc's 'warning: value computed is not used'. */