Prefer `rootDir` over `include`

Because you can use it on the command line to `--init`:

```ts
npx tsc --init --rootDir src outDir lib
```

Locate the documentation section covering `tsc --init`, `rootDir`, and `include`, then review the surrounding explanation and command example. Update the guidance to prefer `rootDir` where appropriate and ensure the shown command is accurate; the issue is done when the documentation clearly reflects that recommendation.

Written by the indexing model from the issue text.