Behavior requires documentation: Pipeline can't run before fusion

When I was prototyping a portable runner, I tested using a trivial pipeline:
```

PCollection result = pipeline.apply("impulse", Impulse.create());
PAssert.thatSingleton(result).isEqualTo(new
byte[]{});

```

When I tried traversing this pipeline, I found that the URN for the transform PAssert$0 was strangely missing. Turns out GreedyPipelineFuser solves this problem. I don't know why this is so, but I think this behavior should at least be documented in the [runner authoring guide](https://beam.apache.org/contribute/runner-guide/#pipeline) to save future runner authors some debugging.

Imported from Jira [BEAM-6762](https://issues.apache.org/jira/browse/BEAM-6762). Original Jira may contain additional context.
Reported by: ibzib.