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 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=<pattern>

Signed-off-by: Minwoo Im <[email protected]>

HOWTO.rst

@@ -2896,7 +2896,9 @@ with the caveat that when used on the command line, they must come after the
                 **read**
                         Use Read commands for data verification
                 **compare**
-                        Use Compare commands for data verification
+                        Use Compare commands for data verification.  This option is only valid with
+                        specific pattern(s), which means it *must* be given with `verify=pattern` and
+                        `verify_pattern=<pattern>`.
 
 .. option:: sg_write_mode=str : [sg]
 

fio.1

@@ -2686,7 +2686,9 @@ Specifies the type of command to be used in the verification phase. Defaults to
 Use Read commands for data verification
 .TP
 .B compare
-Use Compare commands for data verification
+Use Compare commands for data verification.  This option is only valid with
+specific pattern(s), which means it *must* be given with `verify=pattern` and
+`verify_pattern=<pattern>`.
 .TP
 .RE
 .RE