docs/memory.txt: Fix document on MMIO operations

.impl.valid should be .impl.unaligned and the description needs some
fixes.

.old_portio is removed since commit b40acf99b (ioport: Switch
dispatching to memory core layer).

Signed-off-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
