v3.2: Clarify JSON-compatible YAML

[handrews]

I'm stuck at home bored and getting over a cold, so have a PR as I distract myself from missing holiday plans!

This fixes:

• YAML JSON Schema is not enough for roundtrip-safe #2951 (the YAML "JSON schema ruleset" turns out to allow values that can't be expressed in JSON which is a problem for us)
• JSON Schema for YAML usage #4643 (more about how the YAML spec is hard to comprehend than anything else, but it demonstrates problems with our existing text)

Everything in this area is a RECOMMENDED (== SHOULD) so we have some leeway. I have tightened what is recommended but acknowledged previous guidance and simply said that depending on anything allowed by the old guidance but not by the new is NOT RECOMMENDED. I suspect most tools never even noticed, TBH.

• no schema changes are needed for this pull request

[mikekistler]

Looks good. 👍

[handrews]

After further discussions with @hudlow the character set stuff may not be needed regarding the Example Object, so it's worth considering whether that part needs to be here at all, or whether what's in the JSON and YAML RFCs is sufficient (it probably is).

[handrews]

@hudlow@mikekistler I have removed the bit about character encodings (it is truly not needed, after getting a night of sleep- I've been under the weather, please forgive the confusion) and have slimmed down the explanation as much as I can while still conveying that there's a bit of a danger zone that was technically previously recommended, but was never really intended to work.

[mkistler]

Looks good! 👍

[handrews]

@mkistler@mikekistler GitHub won't let me merge unless you approve as @mikekistler

[lornajane]

Indeed, much clearer

[mikekistler]

Looks good. 👍