Clarify $expand=NavSingle/$count and $expand=*/$count

Section 4.8 only defines the /$count segment for collection-valued paths.

Section 5.1.2 does not state a restriction to collection-valued navigation properties for $expand=NavProp/$count and similar paths.

We could allow /$count for all kinds of paths, returning 1 for single-valued navigation paths to an existing resource, and returning 0 for single-valued navigation paths that would return 204 or 404 without the /$count suffix.

Building on that $expand=*/$count could be defined to return the count of related entities for all navigation properties.