Add some documentation for Reader trait.

I saw #3317 go by, and figured I'd give it a bit of a start. I'd like some feedback on what I've done here before I try to document some more.

1. I thought it'd be good to include example blocks even if there are no examples, gives a good marker for the future, and makes sure the markup stays pretty consistent between doc sections.

2. All the usual details. Is my syntax correct? Descriptions fair? All the other stuff.

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