Requests badly documented

[olaf-k]

There are two issues regarding documentation on this topic:

• one that is recurring (see here for example) about having to explicitly set expectedResponseType to json.
  @jakub-g opened Make expectedResponseType == "json" by default in aria.core.IO:asyncRequest? ariatemplates#1016 but in the meantime we should make this clear in the documentation.
• cross-domain requests using the flash object does not propagate headers. While this has to be still investigated, the documentation should reflect the current limitation.