[API] Document Total-Count response header

[iansk]

Document the Total-Count response header.

It contains the number of records available from an end point. While one can simply iterate over the results until pulling a null set, there's the possibility that empty results could be returned due to a server failure and not because all records had been returned. That might be addressable by checking the HTTP status code associated with each request but using Total-Count brings multiple benefits - as long as it's known to work and wasn't omitted for a reason.

It's only returned by end points that tend to produce large data sets, but of course this is where it is needed the most. (work with engr to get a list of endpoints that return it).