Return the position of the encoding in the XML declaration at point.
If there is a well-formed XML declaration starting at point and it
contains an encoding declaration, then return (START . END)
where START and END are the positions of the start and the end
of the encoding name; if there is no encoding declaration return
the position where and encoding declaration could be inserted.
If there is XML that is not well-formed that looks like an XML
declaration, return nil. Otherwise, return t.
If LIMIT is non-nil, then do not consider characters beyond LIMIT.
(fn &optional LIMIT)