Document PARTITION BY KEY

KB does not explain PARTITION BY KEY. It is only mentioned on:
https://mariadb.com/kb/en/partitioning-types-overview/
but there is no separate page like with other partitioning types explaining how it works and what is the full syntax with examples. The syntax is briefly mentioned in CREATE TABLE article.

We have to use MySQL manual instead:
https://dev.mysql.com/doc/refman/5.7/en/partitioning-key.html