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>