attribute limits documentation

Elastic APM Node.js Agent has a [configuration option](https://www.elastic.co/guide/en/apm/agent/nodejs/current/configuration.html#long-field-max-length) to limit the size of the fields sent to APM server.

OpenTelemetry envvar specs does have similar options for logs and traces signals:
- https://opentelemetry.io/docs/specs/otel/configuration/sdk-environment-variables/#attribute-limits
- https://opentelemetry.io/docs/specs/otel/configuration/sdk-environment-variables/#span-limits
- https://opentelemetry.io/docs/specs/otel/configuration/sdk-environment-variables/#logrecord-limits

Is is not a direct correspondence and there is also things to take in consideration when talking about field limits. See https://github.com/elastic/opentelemetry/pull/164#discussion_r2021972199

Checklist

- [ ] check comment about ES truncating fields and other ingestion features that may affect
- [ ] add documentation into https://github.com/elastic/opentelemetry/blob/main/docs/_edot-sdks/nodejs/configuration.md
- [ ] check if theres is a migration from APM Agent config option to these OpenTelemetry options