Backfilling is now supported. Updating documentation.

It works with `!wa`, not with `!whatsapp`

* Allow the mautrix whatsapp relaybot to be enable with a variable

This allows a user to enable the relaybot by setting a variable in
`vars.yml` in the same way that the mautrix signal relaybot is
configured.

* Correct default values for mautrix whatsapp relaybot variables

* Add documentation for using the relaybot with mautrix whatsapp

* Adjust variable names to better reflect what they do

* Set default variables properly and use to_json in template

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)".