Editorial: update usage of the MIME Sniffing Standard

This follows whatwg/mimesniff#58 by referencing
the definitions for JavaScript and JSON MIME type that now live in MIME
Sniffing. It also follows whatwg/mimesniff#36 by
using the terms "valid MIME type string" and "valid MIME type string
without parameters" instead of their non-string counterparts that
previously appeared. Finally, it updates the terms "explicitly supported
XML/JSON type" to include the word "MIME", like other MIME type group
definitions now do.