Add reference about the old parameter 'internal' parameter in 'setData()' method

[Dumluregn]

What is the purpose of this pull request?

To fix inconsistent API Docs about setData() method.
It was caused by maintaining backward compatibility, as mentioned above:

ckeditor4/core/editor.js

Lines 1106 to 1108 in 52ec0b1

	* Note: In **CKEditor 4.4.2** the signature of this method has changed. All arguments
	* except `data` were wrapped into the `options` object. However, backward compatibility
	* was preserved and it is still possible to use the `data, callback, internal` arguments.

Still, as using internal parameter is valid, I added reference to it.

Closes #3417.

[f1ames]

Rebased and retargeted onto latest master.

[f1ames]

👍