docs: update docs for verify_mode=compare of io_uring_cmd

docs: update docs for verify_mode=compare of io_uring_cmd

Add missing limitation of verify_mode=compare in io_uring_cmd
ioengine.  Data verification with NVMe COMPARE command has been
introduced in Commit 6170d92a61da ("io_uring: Support Compare command
for verification") and this should have documented COMPARE command only
supports in case of data pattern verification.

The two more options should be with --verify_mode=compare.
	verify_mode=compare
	verify=pattern
	verify_pattern=<pattern>

Signed-off-by: Minwoo Im <minwoo.im@samsung.com>