[activitystreams] Issue: Linking to RFC 3339 for duration is dubious marked as i18n

r12a has just labeled an issue for 
https://github.com/w3c/activitystreams as "i18n":

== Linking to RFC 3339 for duration is dubious ==
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[This issue has support from others in the i18n WG.]

https://www.w3.org/TR/activitystreams-vocabulary/#dfn-duration

> The value MUST be expressed as an [RFC3339] (ISO 8601) duration 
(e.g. a period of 5 seconds is represented as "PT5S"). 

The reference is a little problematic here, since RFC 3339 describes a
 profile of ISO 8601 for timestamps, and explicitly states that

> Date and time expressions indicate an instant in time. Description 
of time periods, or intervals, is not covered here.

(The description presumably intended at is in the apparently 
non-normative ABNF for ISO 8601 proposed in  Appendix A of RFC 3339.)

Perhaps the definition of xsd:duration would be a better reference?

HTML also includes a 
[duration](https://www.w3.org/TR/2016/WD-html51-20160602/infrastructure.html#durations)
 construct. That one references ISO 8601 in passing. But 
`xsd:duration` is probably the more thorough reference.

See https://github.com/w3c/activitystreams/issues/339