Simplify buck query documentation (Part 1 or 2)
Simplify buck query documentation (Part 1 or 2)

Summary:
- Respond to feedback that the Buck query documentation is too verbose and contains information that is not useful.
- This change covers the introductory material in the topic.
- Remove Backus-Naur Form (BNF) and unnecessarily complex discussion of lexical syntax.
- Simplify discussion of quoting.
- Simplify description of set operators.
- Link to Wikipedia article on first use of //transitive closure//.
- Other edits for clarity and readability.

Reviewed By: cjhopman

fbshipit-source-id: d89974941d

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