Align text description for fields of format duration

[PedroDiez]

Problem description

Within #137 it has been agreed that the fields with format duration follow the format as defined in duration format proposed by OpenAPI Initiative.

Taking this as a baseline, it is proposed to align some text description for the fieldds marked with "duration" format so as fields using this format has an aligned text when used in an API specification

In short, it is doing something analogous as in Issue #14 / PR #55.

Possible evolution
Add in CAMARA-API-Design-Guide.md, Section 2.2, the following text:

- If the format of a string is duration, the following sentence must be present in the description:
  - It must follow [RFC 3339](https://datatracker.ietf.org/doc/html/rfc3339#appendix-A) for duration.

Therefore, we align the text to be present in any duration format field

The proposal is aligned with Issue #137:

RFC Reference -> https://www.rfc-editor.org/rfc/rfc3339.html#appendix-A

Alternative solution
N/A

Additional context
N/A