Document skip-test-db option in other KB pages

Based on MDEV-12645 the option skip-test-db is introduced, but never documented quite well in documentation where is introduced https://mariadb.com/kb/en/mysql_install_db/.
Alternate solution should be to create new KB page explaining usage of test[_*] databases with the information what is a test database, why is used and examples of creating test[_*] derivatives and ignoring security (privileges) in such environment.
Also the direct consequence of this change should be documented in other pages, like https://mariadb.com/kb/en/grant/ (there is no sentence saying that test[_*] databases are exceptions).
This way any link to privilege or grant found in other KB articles will exclude test[_*] database from examples used by specific KB article (example https://mariadb.com/kb/en/previous-value-for-sequence_name/)