vdevprops: remove unused slow_io defaults, fix documentation

Remove the unused DEFAULT_SLOW_IO_N and DEFAULT_SLOW_IO_T defines
from zfs_diagnosis.c.  Unlike the checksum and I/O thresholds, the
slow_io_n and slow_io_t properties must be manually opted in and
have no built-in defaults.  The defines were misleading.

Update the vdevprops man page to clarify that slow_io_n and
slow_io_t must be manually set, and that the documented defaults
(10 errors in 600 seconds) apply only to checksum and I/O events.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <alexander.motin@TrueNAS.com>
Signed-off-by: Christos Longros <chris.longros@gmail.com>
Closes #18359

cmd/zed/agents/zfs_diagnosis.c

@@ -48,8 +48,6 @@
 #define	DEFAULT_CHECKSUM_T	600	/* seconds */
 #define	DEFAULT_IO_N		10	/* events */
 #define	DEFAULT_IO_T		600	/* seconds */
-#define	DEFAULT_SLOW_IO_N	10	/* events */
-#define	DEFAULT_SLOW_IO_T	30	/* seconds */
 
 #define	CASE_GC_TIMEOUT_SECS	43200	/* 12 hours */
 

man/man7/vdevprops.7

@@ -142,7 +142,17 @@ If the property is only set on the top-level vdev, this value will be used.
 The value of these properties do not persist across vdev replacement.
 For this reason, it is advisable to set the property on the top-level vdev -
 not on the leaf vdev itself.
+The
+.Sy slow_io_n
+and
+.Sy slow_io_t
+properties must be manually set to enable slow I/O diagnosis;
+there are no built-in defaults.
 The default values for
+.Sy checksum_n , checksum_t , io_n ,
+and
+.Sy io_t
+on
 .Sy OpenZFS on Linux
 are 10 errors in 600 seconds.
 For