## What changes were proposed in this pull request?

Docs changes:
- Adding a warning that the backend is experimental.
- Removing a defunct internal-only option from documentation
- Clarifying that node selectors can be used right away, and other minor cosmetic changes

## How was this patch tested?

Docs only change

Author: foxish <ramanathana@google.com>

Closes #20314 from foxish/ambiguous-docs.