claify features links wrt to parent link

[philvarner]

Related Issue(s):

• Clarify parent link in Features context #362
• Features endpoint /collection/{c_id}/items does not state it must have a self, root, parent, and collection link #350

Proposed Changes:

1. Clarify what the parent link should point to
2. Add descriptions of Items endpoints

PR Checklist:

• This PR has no breaking changes.
• This PR does not make any changes to the core spec in the stac-spec directory (these are included as a subtree and should be updated directly in radiantearth/stac-spec)
• I have added my changes to the CHANGELOG or a CHANGELOG entry is not required.

[m-mohr]

This one keeps me a bit puzzled. Everywhere in STAC we use parent, but here it is collection. That's not very consistent. I'm wondering whether we should require parent and collection for consistency? On the other hand, /collections also doesn't have a parent, so maybe it's not needed. Just throwing this out here for thoughts.

[philvarner]

The difference is that this is a link rel from the ItemCollection rather than an Item. The use of collection as the link rel is actually from OAFeat, since the Collection is the parent of all the Items, but not the parent of the ItemCollection (which has no parent).

[m-mohr]

This requirement can't really come from STAC Core as a Collections list does not exist in STAC Core. Same applies for /collections/{collectionId}/items. I'm also confused why we require root, but not parent? I'm not saying we should require both, more asking why is it inconsistent? Shouldn't it either require or not require both?

[philvarner]

Agree about From not being Core -- I'll fix that.

Same for /items -- the root Catalog is the parent of all the Collections, but not the "CollectionCollection" returned by /collections, so we don't use parent here. Presumably we could add the equivalent of the collections link rel in /items, but it's always going to be the same as root (I think?) and I don't know what link rel name would be -- catalog?