Improve comment for struct BufferDesc.

Add a note that per-buffer I/O condition variables currently live
outside the BufferDesc struct.  Follow-up for commit d8725104.
Reported-by: Julien Rouhaud <rjuju123@gmail.com>
Discussion: https://postgr.es/m/20210311031118.hucytmrgwlktjxgq%40nol

src/include/storage/buf_internals.h

@@ -174,6 +174,10 @@ typedef struct buftag
  * Be careful to avoid increasing the size of the struct when adding or
  * reordering members.  Keeping it below 64 bytes (the most common CPU
  * cache line size) is fairly important for performance.
+ *
+ * Per-buffer I/O condition variables are currently kept outside this struct in
+ * a separate array.  They could be moved in here and still fit within that
+ * limit on common systems, but for now that is not done.
  */
 typedef struct BufferDesc
 {