[YAML] Better IO documentation.

[robertwb]

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[robertwb]

R: @Polber

[github-actions]

Stopping reviewer notifications for this pull request: review requested by someone other than the bot, ceding control

[Polber]

Not blocking this PR, but I think it might be worth discussing a standardized format for representing the input and output schema of each transform

i.e. for ReadFromText having something like

schema:
  line: string

This of course is trickier for transforms like ReadFromBigQuery where the schema is dynamic, but having a format for static schemas is a good start.

[robertwb]

Yeah, it'd be nice to be able to specify the expected output (and required input) PCollection schemas. I don't think that's available anywhere programatically. (I don't think it's fixed for many transforms, for better or for worse, but for those that it is it could be useful.)