Clarify zinject(8) explanation of -e

Error injection of EIO or ENXIO simply sets the zio's io_error value,
rather than preventing the read or write from occurring.  This is
important information as it affects how the probes must be used.

Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Olaf Faaland <faaland1@llnl.gov>
Closes openzfs#7172

man/man8/zinject.8

@@ -111,6 +111,9 @@ Specify
 .BR "dtl" " for an ECHILD error,"
 .BR "io" " for an EIO error where reopening the device will succeed, or"
 .BR "nxio" " for an ENXIO error where reopening the device will fail."
+For EIO and ENXIO, the "failed" reads or writes still occur.  The probe simply
+sets the error value reported by the I/O pipeline so it appears the read or
+write failed.
 .TP
 .BI "\-f" " frequency"
 Only inject errors a fraction of the time. Expressed as a real number