Document which event fields dont make sense to be set on a transaction

[marandaneto]

The docs on transaction mention that "Transactions are Events enriched with Span data". Does it imply that all event fields are valid and make sense on transaction? For example, would having exception data make sense on transaction? Currently, it's not clear what is the actual contract of the transaction payload.

Source #221

[krystofwoldrich]

We should list fields that are set on the error event but not for transactions.

Example: IO, exception these are not on transaction.

Update https://develop.sentry.dev/sdk/performance, https://develop.sentry.dev/sdk/event-payloads/transaction/