Update `$schema` for `turbo.json`. (#3789)
The in-editor recommendations for `turbo.json` that come from `$schema`
seem to have fallen behind the API in a few spots. I also took a moment
to update some language for improved clarity.
I also had to shave a few yaks to work on this:
- Updated `docs#dev` task to ensure it generates the `schema.json` when
we run `pnpm run docs`
- Improve deprecation note for environment variables
- Added `errors-only` to `outputMode` doc
