draft man for bdev discard

Just a draft, clearly not in the right place.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>

man/io_uring_enter.2

@@ -1262,6 +1262,69 @@ is a pointer to siginfo_t, if any, being filled in. See also
 .BR waitid(2)
 for the general description of the related system call. Available since 6.5.
 
+.TP
+.B BLOCK_URING_CMD_DISCARD
+Issue a discard command to the block device file.
+.I fd
+should point to a block device file,
+.I addr
+is the offset in bytes to start the operation from, and
+.I addr3
+should be set to the length of the operation in bytes.
+It is an asynchronous equivalent of
+.B BLOCK_URING_CMD_DISCARD.
+It allows multiple parallel discards as well as other operations,
+as a result it may result in races for the data on the disk, and it's the user's
+responsibility to take care of it. Furthermore, we only do the best effort page
+cache invalidation, the user has to make sure there are no other inflight
+requests modifying or reading the range, otherwise it might lead to stale
+page cache and data inconsistencies.
+
+Available since 6.1*.
+
+.TP
+.B BLOCK_URING_CMD_WRITE_ZEROES
+Issue a write zeroes command to the block device file.
+.I fd
+should point to a block device file,
+.I addr
+is the offset in bytes to start the operation from, and
+.I addr3
+should be set to the length of the operation in bytes.
+It is an asynchronous equivalent of
+.B BLKZEROOUT.
+It allows issuing multiple parallel operations, as a result it may result in
+races for the data on the disk, and it's the user's responsibility to take care
+of it. Furthermore, we only do the best effort page cache invalidation, the
+user has to make sure there are no other inflight requests modifying or
+reading the range, otherwise it might lead to stale page cache and data
+inconsistencies.
+
+Available since 6.1*.
+
+.TP
+.B BLOCK_URING_CMD_WRITE_ZERO_PAGE
+Issue a write request with the payload consisting of zeroes to the block
+device file.
+.I fd
+should point to a block device file,
+.I addr
+is the offset in bytes within the disk to start the operation from, and
+.I addr3
+should be set to the length of the operation in bytes.
+It is an equivalent of
+.B BLOCK_URING_CMD_WRITE_ZEROES
+but doesn't require hardware support and could be less efficient.
+It allows issuing multiple parallel operations, as a result it may result in
+races for the data on the disk, and it's the user's responsibility to take care
+of it. Furthermore, we only do the best effort page cache invalidation, the
+user has to make sure there are no other inflight requests modifying or
+reading the range, otherwise it might lead to stale page cache and data
+inconsistencies.
+
+Available since 6.1*.
+
+
 .PP
 The
 .I flags