Sampler.ShouldSample has wrong parameter descriptions.

[Oberon00]

Late follow-up from #634.

In https://github.com/open-telemetry/opentelemetry-specification/blob/14b5b6a/specification/trace/sdk.md#shouldsample, the spec has some confusing or even wrong wording when it describes the parameters, namely:

• SpanContext of a parent Span.:

  • Typically extracted from the wire.

    This is not true, since samplers are called for all child spans, not only for (local) root spans. A parent is often a locally created Span.

  • Can be null.

    Normally we use an empty/invalid SpanContext everywhere instead of null. This was actually changed in Java in Sampler.shouldSample can get a parent SpanContext of null opentelemetry-java#1487

• TraceId of the Span to be created. It can be different from the TraceId in the SpanContext. Typically in situations when the Span to be created starts a new Trace.

  I don't think this can be different. If there is any valid parent trace ID, then it must match the newly created Span. If the Span where to start a new Trace, the parent context would be empty (or null with the current wording).

Marking this as P3 since I think the text in question is not really normative. But it's still wrong, so I think it would be desirable to fix this before GA.

CC @alolita