Document how to create encryption keys for file_key_management plugin

The documentation for the file_key_management plugin doesn't currently say how to generate encryption keys:

https://mariadb.com/kb/en/mariadb/data-at-rest-encryption/#file_key_management-plugin

The correct command appears to be shown here:

https://mariadb.com/resources/blog/table-and-tablespace-encryption-mariadb-101

e.g.:

> openssl enc -aes-256-ctr -k mylong2016secret@keyfor35fun -P -md sha1

salt=9265402E0907A5D4

key=55101B33D507041805AF49B95BBAE995B4218C5054D2DA2852C5F0042B837FC3

iv =C7040FF9DB066043D16ADBEC4F18053F

However, this more recent blog post from Pythian suggests a slightly different command:

openssl enc -aes-256-cbc -P -md sha1

https://www.pythian.com/blog/data-encryption-rest/

We should clarify if a specific command is correct.