Update zfs-events.5 with info from PSARC 2009/497

Update zfs-events.5 with info from PSARC 2009/497 regarding ereport fields.
Also updates ZIO_STAGE_* and ZIO_FLAG_* descriptions to match current source.

Reviewed by: loli10K <ezomori.nozomu@gmail.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Elling <Richard.Elling@RichardElling.com>
Closes #8057