Clarify sort

[m-mohr]

From @ycespb at radiantearth/stac-browser#431

StacBrowser sends sorting requests to the server (for collection search) using property names such as "properties.title" while these paths do not exist in the STAC JSON representation of the collection as "properties" does not exist. The path "title" does exist. Similarly, properties.datetime is used as sorting key for collections. Should "extent/temporal/interval" not be used instead in the requests to sort by date ? Maybe the explanation in https://github.com/stac-api-extensions/collection-search ("it works as it does for Items") could be more explicit if the intention is to use the same paths even when they do not really exist in the response...

This need to be worked out better in the Collection Search spec and then be adapted in STAC Browser, too.