added path to the correct vars.yaml in matrix-synapse

This is a new feature of Synapse v1.1.0.

Discussed in #145 (Github Pull Request).

We've had some people get confused into installing
Matrix Corporal and having pain with that.

With this documentation change, we try to make it clearer
that it's an advanced feature not to be touched unless
you know what you're doing.

On a similar note, we also make sure other things are properly
labeled as "(optional)" and/or "(advanced)".