State explicitly if Content-Length is required, and if so, for which request types it is required

The procotol spec (2013-03-12) shows some examples in section 9.6.210.6.2 Batch Request Body, which include Content-Length.

But no examples or requirements are provided for the use of Content-Length elsewhere.

We should state explicitly when Content-Length must be specified (if at all).

Note: Content-Length is particularly valuable for clients accessing large Stream values, as it allows the client applications to provide progress meters in the UI.

It is less useful for clients accessing entity sets, as the server may (via the use of nextLink) break up a response into multiple pages. If clients could indicate via a preference that they wish to receive an entire entity set (or delta link contents) in a single HTTP response, then Content-Length would be quite useful for entity sets and delta links.