Fix typos and minor documentation errors in both memory.h and
docs/memory.txt.

Also add missing documentation formatting tags to transaction
functions.

Reviewed-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Ademar de Souza Reis Jr <areis@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>