Claify Transaction Extension behavior

[philvarner]

Related Issue(s):

• Transaction Extension needs clarification on behaviors #285

Proposed Changes:

1. Clarify that PUT and PATCH cannot change the collection an item is in
2. Clarify allowed HTTP status codes
3. Allow partial Item and partial ItemCollection containing partial Items as POST input
4. Allow partial Item containing partial Items as PUT input (but the server has to be able to derive a valid Item from it to return from the GET endpoint, e.g., by populating the id and collection fields or adding links)

PR Checklist:

• This PR is made against the dev branch (all proposed changes except releases should be against dev, not master).
• 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.

[philvarner]

I think this missed being deleted when this block was copied from the Item schema

[philvarner]

Filed:

• Part 4: Should PUT or PATCH be allowed to change the id? opengeospatial/ogcapi-features#678

[philvarner]

Filed:

• Stylize all conformance class name references with italics instead of code #274

[philvarner]

@m-mohr ready for re-review