Issue in span context description

[austinlparker]

Currently on https://opentelemetry.io/docs/concepts/signals/traces/#span-context we describe the span context as "Span context is an immutable object on every span that contains the following:"

This however is not the case at a proto level -- the span context is not a separate object, it's a set of fields. We should clarify or write around this somehow to be more accurate.

[cartermp]

Should we? I don't think most people are going to worry about what it's like at the protocol level.