Update doc for JsCreateString(Utf16)
Update doc for JsCreateString(Utf16)

Mention that JsCreateString, and JsCreateStringUtf16 require an active script context.
See Microsoft/ChakraCore-wiki#41.

Microsoft/ChakraCore,Microsoft/ChakraCore,mrkmarron/ChakraCore,Microsoft/ChakraCore,Microsoft/ChakraCore,Microsoft/ChakraCore,mrkmarron/ChakraCore,mrkmarron/ChakraCore,mrkmarron/ChakraCore,Microsoft/ChakraCore,mrkmarron/ChakraCore,mrkmarron/ChakraCore,mrkmarron/ChakraCore