H2 Database Repository should be moved from default directory

In the admin-guide it should be mentioned that property:

nifi.database.directory

should also be changed to some other folder from its default location

<root-level-nifi-dir>/database_repository

It should be marked with an asterisk according to this post:

https://community.hortonworks.com/articles/7882/hdfnifi-best-practices-for-setting-up-a-high-perfo.html