Explain that nullable fields in shapes are also optional

[barillari]

Warn shape users that the typechecker conflates optional and nullable.

[daviddoran]

You could mention that Shapes::idx can be used to access a possibly missing or null field safely?

[fredemmott]

See also #374 #375

?? is usable instead of Shapes::idx() and largely preferred externally (as it's not hack-specific and is just as safe)

[fredemmott]

Merging in as-is - agree with @daviddoran, but that can be a separate PR