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

diff --git a/Documentation/block/queue-sysfs.txt b/Documentation/block/queue-sysfs.txt
index 87abf1a..14235e7 100644
--- a/Documentation/block/queue-sysfs.txt
+++ b/Documentation/block/queue-sysfs.txt
@@ -58,6 +58,20 @@
 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