Add rotation doc for credhub_ca and credhub_cli_user_password
Add rotation doc for credhub_ca and credhub_cli_user_password

[#160988099](https://www.pivotaltracker.com/story/show/160988099)

Co-authored-by: Florian Nachtigall <da0e822b67ef154a118f514785d782d9f0817746@sap.com>

cloudfoundry/docs-bosh