Document missing fields from span interface

[marandaneto]

The docs on spans don't cover the span_id/parent_span_id/sampled fields and potentially some others. What is the list of fields required/allowed on spans?

https://develop.sentry.dev/sdk/event-payloads/span/

Source #221

[rhcarvalho]

This is a rendering bug, see #170.

The source code has the includes for span_id and parent_span_id:

Note that sampled is not part of the spec intentionally, otherwise it would be a field that would be always true, because sampled: false means do not send it.
Therefore, sampled is an internal field that tracks the in-memory state of the span, but is not part of its JSON serialization when sending to Sentry.

Closing this as a duplicate of #170, since this will be fixed once we fix that one.