Document new query parameter support for Core Command and Device SDKs

Support was recently added to Core Command and the Device SDKs to handle query parameters for command REST requests. These parameters are copied "as is" from a Core Command REST request to the request made to the device service, which then adds them to the attributes map passed to the device service ProtocolDriver.

This issue covers adding documentation for Core Command and the Device Service SDKs to explain how these query parameters are handled.

This documentation should also mention that query parameters with the prefix "ds-".

For reference see the following PRs:

* https://github.com/edgexfoundry/edgex-go/pull/1571
* https://github.com/edgexfoundry/device-sdk-go/pull/336