docs: Fix incorrect otel future flag bypass claim (#12047)
## Summary
- Fixes inaccurate documentation that claimed environment variables and
CLI flags could enable observability without the
`futureFlags.experimentalObservability` flag
- The code enforces the future flag for all config sources: `turbo.json`
config is silently dropped at parse time, and env var/CLI config is
rejected with a hard error in the run builder
