docs/misc: xen-command-line: Rework documentation of the option 'serrors'

docs/misc: xen-command-line: Rework documentation of the option 'serrors'

The current documentation is misleading for a few reasons:
    1) The synchronization happens on all exit/entry from/to the guest.
       This includes from EL0 (i.e userspace).
    2) Trusted guest can also generate SErrors (e.g. memory failure)
    3) Without RAS support, SErrors are IMP DEFINED. Unless you have a
    complete TRM in hand, you can't really make a decision.
    4) The documentation is written around performance when this is not
    the first concern.

The documentation is now reworked to focus on the consequences of using
serrors="panic" and avoid to go in details on the exact implementation.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>
Release-acked-by: Juergen Gross <jgross@suse.com>