更改rtdef.h中RT_ALIGN和RT_ALIGN_DOWN的注释

Author: liuyucai

include/rtdef.h

@@ -261,6 +261,7 @@ typedef int (*init_fn_t)(void);
  * @def RT_ALIGN_DOWN(size, align)
  * Return the down number of aligned at specified width. RT_ALIGN_DOWN(13, 4)
  * would return 12.
+ * @note align Must be an integer power of 2 or the result will be incorrect
  */
 #define RT_ALIGN_DOWN(size, align)      ((size) & ~((align) - 1))