docs: allow_extra_fields

[jchavarri]

Adds docs for json.allow_extra_fields.

[andreypopp]

The only catch is that json.allow_extra_fields is the default for browser ppx — we don't check for extra fields there. 😬

[jchavarri]

Ah, interesting! Would it make sense to consolidate? If we do so, maybe we can make ignoring extra fields the default on both native and js, and add a decorator @json.strict. I think that allowing extra fields is a more common use case. Wdyt?

[andreypopp]

Sounds like a good idea.