Signed-off-by: Suguru Hirahara <acioustick@noreply.codeberg.org>

Overall the playbook uses the expression "Add the following configuration to your `inventory/host_vars/matrix.DOMAIN/vars.yml` file:" with the heading "Adjusting the playbook configuration" for sections to explain what to be added as variables

Signed-off-by: Suguru Hirahara <acioustick@noreply.codeberg.org>

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)".