Document MDEV-12179 (Per-engine mysql.gtid_slave_pos tables)

MDEV-12179 has been pushed into 10.3 tree to be released with 10.3.1.

I see that somebody has already documented its existence of the feature and new system/status variables in the KB, but it's not enough, it needs to be documented in terms of operation, with at least minimal functional examples to show what one needs to make it work:

• when the variable value becomes effective – after server restart, after slave restart, upon receiving certain events, etc.;
• in which cases the extra tables are created automatically and in which they are not;
• when user needs to create the tables manually, if ever;
• when user needs to populate the table manually, and if so, with what;
  etc.

There is a test rpl.rpl_mdev12179, but it doesn't help, the logic appears seemingly random.