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.

Reviewed-by: Yonggang Luo <luoyonggang@gmail.com>
Part-of: <!19593>