docs: drop custom envvar directive

The custom envvar directive is strange. It overloads the global namespace, where there's already a well-known envvar directive, but the envvar role won't resolve to envvars defined using the custom directive.

It's not really critical to declare the type and default value here, and many descriptions already include this. Let's just use the plain one, and let the description take care of the details where needed.

While we're at it, clean up a bunch of other minor envvar related stuff.