Row level lock

On the documentation
http://www.postgresql.org/users-lounge/docs/7.0/postgres/mvcc4646.htm what
does mean "...Postgres doesn't remember any information about modified rows
in memory and so has no limit to the number of rows locked without lock
escalation."?

Following you find "However, take into account that SELECT FOR UPDATE will
modify selected rows to mark them and so will results in disk writes." Why
does it result in disk writes?

Paulo Siqueira