Documentation - switching to row based replication and data loss

If you are running gh-ost on master (ie --allow-master-master, --allow-on-master) with anything other than row based binary logs the switch to row based needs to be handled carefully (more carefully than currently documented)

We found that many of our connections to mariadb are very long lasting (we recycle the connections a lot) - after a great deal of experimentation after some significant data loss it was discovered that the connections that were made prior to the switch to row based binary logging retained the mixed setting until closed. Transactions that took place on these connections are not shuttled into the building _gho table

These binary logs entries are silently ignored by gh-ost