Parser spec notes - mention JSON manglers

[dmsnell]

See serializeAttributes() where we mangle the JSON attributes in order to provide additional safety against transformations that WordPress might further take.

That is, it's not just "spec JSON" even though it should still be spec compliant. Also, I don't think we've specifically chosen RFC 7159 as our implementation so it may be worth only documenting the level of ambiguity we actually have: we resort to the platform implementation of the JSON parser and serializer.

also see…

• Block API: Replace JSON-escaped quotation mark with unicode escape sequence WordPress/gutenberg#6619
• Element: Avoid double-escaping valid character references WordPress/gutenberg#6620