malloc.h: explicitly suggest malloc_info() instead of mallinfo().

I've also sent a patch to the man7.org man page.

Change-Id: I715eacf129abe61562c6d79c4eda03bab31f9c77

libc/include/malloc.h

@@ -125,7 +125,8 @@ struct mallinfo {
 
 /**
  * [mallinfo(3)](http://man7.org/linux/man-pages/man3/mallinfo.3.html) returns
- * information about the current state of the heap.
+ * information about the current state of the heap. Note that mallinfo() is
+ * inherently unreliable and consider using malloc_info() instead.
  */
 struct mallinfo mallinfo(void);