doc: fix the descriptions of the log_prio option

doc: fix the descriptions of the log_prio option

Commit 03ec570f6e57 ("fio: Introduce the log_prio option") added the
description of the log_prio option to fio.1. However, the description
was wrong. It mentioned that the log_prio option would change the number
of fields in the Log File Format, but actually it does not do so. Fix
the description.

Also, the commit did not update HOWTO.rst for the log_prio option. To
keep HOWTO.rst same as fio.1, add the missing descriptions to HOWTO.rst.

Fixes: 03ec570f6e57 ("fio: Introduce the log_prio option")
Signed-off-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Link: https://lore.kernel.org/r/20240829085826.999859-7-shinichiro.kawasaki@wdc.com
Signed-off-by: Vincent Fu <vincent.fu@samsung.com>