Add partial updates to MyRocks limitations

https://www.percona.com/doc/percona-server/LATEST/myrocks/limitations.html does not mention partial updates which is another limitation of MyRocks compared to InnoDB.

That is, MyRocks is currently missing an equivalent of the functionality described here: https://mysqlserverteam.com/mysql-8-0-optimizing-small-partial-update-of-lob-in-innodb/