jQuery.ajax: rewrite Data Types section

I've rewritten the **Data Types** section because almost every single detail in the whole thing was utterly wrong.

I had some difficulty cleanly writing the paragraph about JSONP, and perhaps it isn't as clear as the others, but it's definitely an improvement on what came before, so I can live with it.

More importantly, there remains significant redundancy of content between the documentation of the `dataType` parameter and the **Data Types** section lower down the page; it may be a good idea to consolidate the two. I'm leaving that for another day, though; removing mass misinformation and slimming down the **Data Types** section in the process is at least a step in the right direction.

Full commit message copied below for ease of reading...

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The "Data Types" was riddled with errors and confusions,
to the point that it was nothing but harmful.

I think this commit fixes all of them. I list them below:

* The first paragraph claimed that all non-XML responses
  are treated as text. This is untrue and contradicted
  other paragraphs in the same section.

* The third paragraph claimed that XML responses are
  passed to the success handler with no 'processing'. This
  is untrue (and contradicts the first paragraph); in fact
  they are parsed using $.parseXML.

* The third paragraph stated that text and XML responses
  were passed to the success handler 'through the
  responseText or responseXML property of the jqXHR
  object'. While this is not untrue, it is confusing
  because it neglects the more obvious and useful way by
  which these responses are passed to success and .done
  handlers: directly, as the first argument of those
  handlers.

* The fourth paragraph was entirely untrue. It claimed that
  the Content-Type returned by the server 'must' match the
  dataType specified in the settings passed to $.ajax. In
  reality, if a dataType is passed to $.ajax, the
  Content-Type returned by the server is ignored entirely.

* The fifth paragraph claimed that JavaScript embedded in
  HTML responses would be executed automatically before
  passing the response to the success handler. This is
  untrue. (#482)

* The fifth paragraph additionally claimed that when the
  'script' dataType is specified, the response from the
  server is not passed to the success handler. This is
  also untrue.

* The sixth paragraph contained the confused claim that the
  'json' dataType "uses jQuery.parseJSON() when the browser
  supports it". This is too confused to even be false; it
  is meaningless. jQuery.parseJSON is a jQuery method that
  works regardless of whether the browser provides a native
  implementation of JSON.parse, and so it is impossible for
  the browser to not support it.

* The sixth paragraph additionally contained the questionable
  advice "If the fetched data file exists on a remote server,
  specify the jsonp type instead [of json]." This seems to
  suggest that simply specifying the jsonp dataType will
  magically cause the remote server to return a jsonp response
  instead of a json response. While this may be true for some
  APIs that look for a callback parameter in the request to
  decide whether to serve a JSON or JSONP response, it is
  certainly not universally true.

* The seventh paragraph stated that the server should form a
  valid JSONP response by "prepend[ing] the JSON data with the
  callback name". This is almost but not quite correct, and as
  such is at best useless and at worst actively misleading.

* The ninth paragraph contained the claim that data can only be
  "retrieved from remote servers" when "using the script or
  jsonp data types". Firstly, this is potentially confusing if
  the reader does not understand that the term 'remote servers'
  refers to cross-domain requests, which is a slightly odd usage
  (all servers are remote from the client's perspective!)
  Secondly, even interpreted as intended, it is untrue, since
  $.ajax supports CORS.

This rewrite:

- fixes all of the above issues
- removes first- and second-person pronouns from the copy per
  the style guide

Fixes #482