KB page on System-Versioned tables should explain secure_timestamp variable and when it should be set to YES

KB page on System-Versioned Tables (https://mariadb.com/kb/en/library/system-versioned-tables/) should explain the details about adding row_end column to the primary key implicitly, and consequences of this for the replication setup, including the need to set the value of secure_timestamp variable explicitly.

See MDEV-16370 for one of cases that shows when setting secure_timestamp to YES is needed. Another case is a replication slave that may have to replicate frequent changes for the same row on fast master.