Adopt consistent style in field and code descriptions

[jpmckinney]

The style and apparent audience (e.g. publisher or user) of descriptions in the schema is inconsistent. Compare the Tender.submissionMethod field to the 'unsuccessful' code in awardStatus.

• Update schema descriptions for common field types based on the description templates
• Update schema descriptions for other fields based on the style guide

Some codelist descriptions are explicitly normative and some are implicitly normative (compare tag to Identifier.scheme).

• Update codelist descriptions per codelist description template

[jpmckinney]

This is a bigger project, so moving to 1.2.0 (linked to #826 and #827).

[ColinMaudry]

@jpmckinney The description of this issue has not aged well as the style guide has grown significantly and field and code decriptions may have changed since then. Could you please update the issue description with updated examples of what needs fixing?

I assume this is the part of the style guide that is relevant today: https://ocds-standard-development-handbook.readthedocs.io/en/latest/meta/schema_style_guide.html#field-and-code-descriptions