Clarify Collection ownership / backlink to Actor

[ThisIsMissEm]

From #465:

But then Collections also currently don't contain a backlink to the Actor who manages or owns them.

This started quite a number of tangent comments on that issue, opening this as a discussion place instead for them.

[trwnh]

https://github.com/w3c/activitypub/issues/467 may be a partial duplicate or superset of this issue

see also: https://w3id.org/fep/c7d3

[evanp]

attributedTo, obviously, as for all other types of objects.

[evanp]

https://www.w3.org/wiki/ActivityPub/Primer/Server-Managed_Collections might be a good place to document this issue.

[evanp]

@ThisIsMissEm makes the important point that none of the example collections in the AP or AS2 specifications include an attributedTo property. It would be useful to include these in a future version of either or both specs.