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 #7172