Doc: Update the documentation for row movement behavior across partitions.
Doc: Update the documentation for row movement behavior across partitions.

In commit f16241bef7c, we have changed the behavior for concurrent updates
that move row to a different partition, but forgot to update the docs.
Previously when an UPDATE command causes a row to move from one partition
to another, there is a chance that another concurrent UPDATE or DELETE
misses this row.  However, now we raise a serialization failure error in
such a case.

Reported-by: David Rowley
Author: David Rowley and Amit Kapila
Backpatch-through: 11 where it was introduced
Discussion: 34f89adcdcc4515bf48c3c051d1026a1621d04ff@mail.gmail.com

50wu/gpdb,50wu/gpdb,greenplum-db/gpdb,xinzweb/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,adam8157/gpdb,xinzweb/gpdb,adam8157/gpdb,xinzweb/gpdb,greenplum-db/gpdb,lisakowen/gpdb,lisakowen/gpdb,xinzweb/gpdb,adam8157/gpdb,50wu/gpdb,adam8157/gpdb,greenplum-db/gpdb,50wu/gpdb,xinzweb/gpdb,lisakowen/gpdb,greenplum-db/gpdb,lisakowen/gpdb,xinzweb/gpdb,adam8157/gpdb,adam8157/gpdb,xinzweb/gpdb,lisakowen/gpdb,50wu/gpdb,adam8157/gpdb,lisakowen/gpdb,50wu/gpdb,50wu/gpdb,greenplum-db/gpdb,lisakowen/gpdb,50wu/gpdb,adam8157/gpdb,lisakowen/gpdb,xinzweb/gpdb