KVM: Documentation: Add arm64 KVM_RUN error codes

The API documentation states that general error codes are not detailed, but
errors with specific meanings are. On arm64, KVM_RUN can return error
numbers with a different meaning than what is described by POSIX or the C99
standard (as taken from man 3 errno).

Absent from the newly documented error codes is ERANGE which can be
returned when making a change to the EL2 stage 1 tables if the address is
larger than the largest supported input address. Assuming no bugs in the
implementation, that is not possible because the input addresses which are
mapped are the result of applying the macro kern_hyp_va() on kernel virtual
addresses.

CC: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20201201150157.223625-2-alexandru.elisei@arm.com