JSON: clarify escapes for the @name attribute

All,

The spec says:

    "The content of a string is copied across, and an attribute
type="string" is added to its containing element. Escaped characters
are transformed as necessary; characters and escapes that have no
equivalent XML character (\b, \f, and characters of the form \uxxxx
less than \u0020) are transformed by adding \uE000 to them."

This spec text covers `type="string"`, but we also need to say that
this applies to the value of the `@name` attribute, as the JSON
property name can also contain such "escapes that have no equivalent
XML character".

-Erik