clarify default behavior of `parameterConsistency` or require parameter

[benbellick]

Problem

The JSON schema defines these parameters but doesn't mark them as required:

substrait/text/simple_extensions_schema.yaml

Lines 158 to 168 in 92d2e75

	variadicBehavior:
	type: object
	additionalProperties: false
	properties:
	min:
	type: number
	max:
	type: number
	parameterConsistency:
	type: string
	enum: [CONSISTENT, INCONSISTENT]

The docs define the meaning of the parameters when set, but doesn't specify the behavior when the consistency parameter isn't set.

Fixes

One fix is to just require the parameter. Requiring the parameter could in theory break backwards compatibility. However, the breaking of backwards-compat is a minor issue because implementers will just throw an error at YAML-load time. This approach has the benefit of being explicit

The other option is to just define CONSISTENT as the default behavior when nothing is set.

[benbellick]

Reading the docs a bit closer, parameterConsistency is really only applicable when the last argument contains a type parameter. Thus, I think the correct approach to say that the field is required in this case.

[benbellick]

For the sake of making it easier for implementers, I opted to instead:

1. keep parameterConsistency optional,
2. clarify that the default behavior is CONSISTENT,
3. note that parameterConsistency has no effect unless the last parameter contains a type variable, and
4. recommend that the option be set explicitly if it has an effect.

[benbellick]

As discussed with @vbarua offline, I will also contribute examples to of consistent vs inconsistent to help clarify its meaning.