document that transactions with `unknown` outcomes are ignored in the error rate calculation

https://github.com/elastic/apm/blob/main/specs/agents/tracing-transactions.md#outcome-api says:

> The documentation [for the Outcome API] should clarify that transactions with `unknown` outcomes are ignored in the error rate calculation.

There are two places this is relevant in our docs:

1. our [`transaction.setOutcome()` api docs](https://www.elastic.co/guide/en/apm/agent/nodejs/current/transaction-api.html#transaction-setoutcome)
2. (once the OTel Bridge PR is in) in the OTel Bridge docs somewhere, in reference to creating an OTel span without calling `span.setStatus()`. The default OTel Span status is `UNSET`, which translates to `.outcome = 'unknown'`. To have local-root OTel spans (i.e. those that are translated to Elastic Transactions) be included in error rate calculations in the APM UI, the user should be setting a status on them.