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

[trentm]

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
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.