xen/arm: traps: Rework __do_serror() documentation

xen/arm: traps: Rework __do_serror() documentation

The documentation on top of __do_serror() is trying to describe all the
possibilities to receive an SErrors.

The description of type#2 is quite misleading because receiving an
SError in EL2 after unmasking SError interrupt ({PSTATE, CPSR}.A) does
not necessarily imply the SError were generated by the guest. You also
need to be in a special window (see abort_guest_exist_{guest, end}).

However, for the context of the function it does not matter how we
categorize the interrupts. What matter is to know whether this is a
guest-generated SError.

All the documentation of __do_serror() is now reworked to avoid
misleading information.

Take the opportunity to simplify the code after the forward option has
been dropped.

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