docs: update docs for verify_mode=compare of io_uring_cmd

[minwooim]

Add missing limitation of verify_mode=compare in io_uring_cmd ioengine. Data verification with NVMe COMPARE command has been introduced in Commit 6170d92 ("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=

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1. First line is a commit title, a descriptive one-liner for the change
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5. Signed-off-by: Real Name [email protected]

Reminders:

1. If you modify struct thread_options, also make corresponding changes in
   cconv.c and bump FIO_SERVER_VER in server.h
2. If you change the ioengine interface (hooks, flags, etc), remember to bump
   FIO_IOOPS_VERSION in ioengines.h.

[vincentkfu]

Please update the HOWTO as well.