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
Reported-by: Julien Rouhaud
Discussion:
@@ -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
{
