Requirements: Requirements class' description:: should be rendered as "Description" not "Statement"

[ronaldtse]

A requirements class in ISO has the description key, should be rendered into "Description", not "Statement"

[requirements_class]
.Register management roles and responsibilities
====
[%metadata]
identifier:: /req/roles
subject:: register roles and responsibilities
description:: Roles and responsibilities for a register that conforms with this
document.
requirement:: /req/roles/register-owner/specify-register-specification
...

Current output: