Clarify `UTF-8'

XML Signature spec. and Canonical XML spec. refer 'UTF-8' many
times.  Please clarify which is each UTF-8, 'UTF-8' (with UTF-8
signature, EF BB BF) or 'UTF-8N' (without UTF-8 siganture).

See Table 2 in http://www-4.ibm.com/software/developer/library/utfencodingforms/index.html

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TAMURA Kent @ Tokyo Research Laboratory, IBM