[depr.locale.stdcvt.req] D.18.2 normative references for UCS-2 and UCS-4

[jensmaurer]

CWG in Rapperswil:

"The facet shall convert between UTF-8 multibyte sequences and UCS2 or UCS4"

add "as specified in ISO/IEC 10646-1:1993" for UCS-2 and UCS-4 (add hyphen).
Add "as specified in ISO/IEC 10646" for UTF-8.