zpool-iostat.8: clarify first report shows per-second averages

The first report printed by zpool iostat shows average per-second
values since boot, not cumulative totals. Clarify this in the man
page to avoid user confusion.

Reviewed-by: Tony Hutter <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: Christos Longros <[email protected]>
Closes #18337

man/man8/zpool-iostat.8

@@ -67,8 +67,8 @@ If
 is specified, the command exits after
 .Ar count
 reports are printed.
-The first report printed is always the statistics since boot regardless of
-whether
+The first report printed is always the average per-second
+statistics since boot regardless of whether
 .Ar interval
 and
 .Ar count