Revise the variable/option innodb_encryption_threads

From the documentation (https://mariadb.com/kb/en/mariadb/xtradbinnodb-server-system-variables/#innodb_encryption_threads) it appears that the corresponding threads do key rotation and scrubbing, but not the encryption as such.

Maybe it should be renamed then?

Of course, it's not easy to find a good name because (again, from the description) the same set of threads perform seemingly unrelated operations. Shouldn't it be split into two, one for scrubbing and another one for key rotation?