Ref Guide: Improve DIH docs around encrypting the password

A user pointed me to the below StackOverflow question and suggested we could improve the DIH docs around encrypted passwords a bit. He's right - there is only a side comment that suggests how one could do it and it should have a better section highlighting how to configure it.

https://stackoverflow.com/questions/46819637/how-do-you-encrypt-the-databse-password-used-by-solrs-datainputhandler-dih/46819643#46819643