block: document the 'io_poll_delay' queue sysfs file

This was documented in the original commit, 64f1c21e86f7, but it
never made it into the proper location for queue sysfs files.

Signed-off-by: Jens Axboe <axboe@fb.com>

Documentation/block/queue-sysfs.txt

@@ -58,6 +58,20 @@ When read, this file shows the total number of block IO polls and how
 many returned success.  Writing '0' to this file will disable polling
 for this device.  Writing any non-zero value will enable this feature.
 
+io_poll_delay (RW)
+------------------
+If polling is enabled, this controls what kind of polling will be
+performed. It defaults to -1, which is classic polling. In this mode,
+the CPU will repeatedly ask for completions without giving up any time.
+If set to 0, a hybrid polling mode is used, where the kernel will attempt
+to make an educated guess at when the IO will complete. Based on this
+guess, the kernel will put the process issuing IO to sleep for an amount
+of time, before entering a classic poll loop. This mode might be a
+little slower than pure classic polling, but it will be more efficient.
+If set to a value larger than 0, the kernel will put the process issuing
+IO to sleep for this amont of microseconds before entering classic
+polling.
+
 iostats (RW)
 -------------
 This file is used to control (on/off) the iostats accounting of the