ModSpec instances: the "statement" should just be called `statement::` instead of `description::`

[ronaldtse]

Right now we use an encoding as follows:

[requirement]
.Persistent identifiers do not change throughout the life of the register
====
[%metadata]
identifier:: /req/identifiers/ri-identifier-persistence
subject:: persistent identifier
description:: A persistent identifier within the register shall not change
throughout the lifetime of the register.
guidance:: While the register item pointed to by a persistent identifier may change,
the identifier always points to the same register item.
Even after the decommission of a register, the persistent identifiers for
register items within that register remain.
====

However, this is very confusing because in rendering, the description:: field gets rendered as "Statement":

We should support the label of statement:: instead of description:: (keep description as an alias for compatibility purposes).

Document: https://github.com/metanorma/iso-19135/

[opoudjis]

If you want no differentiation between markup and rendering, you might prefer to use Word. And description is used already instead of statement as a label in classes, by your request. Which means you have already introduced the synonymy.

[ronaldtse]

Thanks @opoudjis for implementing this.