Add more info on PI_CSS to identify nodes
Add more info on PI_CSS to identify nodes

The config setting PI_CSS can be easily used
to identify different privacyIDEA nodes.

The documentation on this has been improved.

Closes #1961

privacyidea/privacyidea,privacyidea/privacyidea,privacyidea/privacyidea,privacyidea/privacyidea,privacyidea/privacyidea,privacyidea/privacyidea