Document how to safely decrypt InnoDB tables

We should document how to safely decrypt InnoDB tables. The documentation doesn't currently say:

https://mariadb.com/kb/en/library/encrypting-data-for-innodb-xtradb/

It looks like the process would go like this:

1.) Set innodb_encrypt_tables=OFF:

SET GLOBAL innodb_encrypt_tables=OFF;

Make sure to also set it in the configuration file.

https://mariadb.com/kb/en/library/xtradbinnodb-server-system-variables/#innodb_encrypt_tables

2.) Make sure innodb_encryption_threads>0 is set:

SET GLOBAL innodb_encryption_threads=4;

https://mariadb.com/kb/en/library/xtradbinnodb-server-system-variables/#innodb_encryption_threads

3.) Make sure that innodb_encryption_rotate_key_age=1 is set:

SET GLOBAL innodb_encryption_rotate_key_age=1;

https://mariadb.com/kb/en/library/xtradbinnodb-server-system-variables/#innodb_encryption_rotate_key_age

4.) Check for individual tables that have explicitly been encrypted:

SELECT TABLE_SCHEMA, TABLE_NAME, CREATE_OPTIONS 

FROM information_schema.TABLES

WHERE CREATE_OPTIONS LIKE '%`ENCRYPTED`=YES%';

5.) For each table that has explicitly been encrypted, explicitly decrypt it:

ALTER TABLE encrypted_tab ENCRYPTED=NO;

6.) Allow the background encryption threads to decrypt all tables. You will know all tables are decrypted when the following query returns 0 rows:

SELECT * 

FROM information_schema.INNODB_TABLESPACES_ENCRYPTION 

WHERE ENCRYPTION_SCHEME != 0 

OR ROTATING_OR_FLUSHING != 0\G