Add documentation note about JSON.parse not being spec compliant

[RedMser]

Part of #66114
Fixes #40394

List all known differences between Godot's JSON parser and the specs, as taken from #66114.
The crashes are not listed here as they are fixed in #66117. Invalid escape sequences are fixed in #66170.

[akien-mga]

Seems good, but I think it would be good enough to mention this once in the class description instead of repeating it in both parse methods.

[RedMser]

Fair point, this makes it more convenient to list all spec-breaking differences as well.

I left out the invalid escape sequences from the docs, as I now opened #66170 to fix it instead. I also left out two very minor parsing differences (null byte at end, and form feed as whitespace) as I don't think they'll cause any problems in real-world applications.

[akien-mga]

Thanks!