[Bug]: Missing documentation for pipeline draining

### What happened?

Going through the documentation for [DoFn](https://beam.apache.org/releases/javadoc/2.46.0/org/apache/beam/sdk/transforms/DoFn.html) I came across the reference to [DoFn.TruncateRestriction](https://beam.apache.org/releases/javadoc/2.46.0/org/apache/beam/sdk/transforms/DoFn.TruncateRestriction.html) which refers to a capability to drain pipelines.
The page links to a[ proposal document](https://docs.google.com/document/d/1NExwHlj-2q2WUGhSO4jTu8XGhDPmm3cllSN8IMmWci8/edit#heading=h.eme0amhbmi73) and yet it is not clear what is the state of this proposal.
There is not mention of the capability to drain pipelines anyhere else in the documentation and all issues that seem to refer to that are related to DataFlow specifically.
If this capability has been implemented it might be best to document it and how it can be used?
Otherwise maybe the documentation should not refer to proposals or features that are not supported across other runners?

### Issue Priority

Priority: 2 (default / most bugs should be filed as P2)

### Issue Components

- [ ] Component: Python SDK
- [X] Component: Java SDK
- [ ] Component: Go SDK
- [ ] Component: Typescript SDK
- [ ] Component: IO connector
- [ ] Component: Beam examples
- [ ] Component: Beam playground
- [ ] Component: Beam katas
- [X] Component: Website
- [ ] Component: Spark Runner
- [ ] Component: Flink Runner
- [ ] Component: Samza Runner
- [ ] Component: Twister2 Runner
- [ ] Component: Hazelcast Jet Runner
- [ ] Component: Google Cloud Dataflow Runner