data-binary.d: clarify default content-type is x-www-form-urlencoded

- Advise user that --data-binary sends a default content type of
  x-www-form-urlencoded, and to have the data treated as arbitrary
  binary data by the server set the content-type header to octet-stream.

Ref: https://github.com/curl/curl/pull/2852#issuecomment-426465094

Closes https://github.com/curl/curl/pull/3085