Annotate certain features as optional/advanced

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)".

docs/configuring-playbook-rest-auth.md

@@ -1,4 +1,4 @@
-# Setting up the REST authentication password provider module
+# Setting up the REST authentication password provider module (optional, advanced)
 
 The playbook can install and configure [matrix-synapse-rest-auth](https://github.com/kamax-io/matrix-synapse-rest-auth) for you.