Script tags now used for most async requests in 4.0

[mgol]

The full logic for whether to use the script tag is implemented by this function:

function canUseScriptTag( s ) {

	// A script tag can only be used for async, cross domain or forced-by-attrs requests.
	// Requests with headers cannot use a script tag. However, when both `scriptAttrs` &
	// `headers` options are specified, both are impossible to satisfy together; we
	// prefer `scriptAttrs` then.
	// Sync requests remain handled differently to preserve strict script ordering.
	return s.scriptAttrs || (
		!s.headers &&
		(
			s.crossDomain ||

			// When dealing with JSONP (`s.dataTypes` include "json" then)
			// don't use a script tag so that error responses still may have
			// `responseJSON` set. Continue using a script tag for JSONP requests that:
			//   * are cross-domain as AJAX requests won't work without a CORS setup
			//   * have `scriptAttrs` set as that's a script-only functionality
			// Note that this means JSONP requests violate strict CSP script-src settings.
			// A proper solution is to migrate from using JSONP to a CORS setup.
			( s.async && jQuery.inArray( "json", s.dataTypes ) < 0 )
		)
	);
}

We should document it.