Rewrite buck clean for readability and usability
Rewrite buck clean for readability and usability

Summary:
- Rewrite introductory paragraphs.
- Explain that user should not try to manually modify `buck-out` dir.
- Replace `--verbose` command with link to **Common Parameters** topic.
- Alphabetize the list of parameters.
- Link to **Cache** section of `.buckconfig` for `--keep-cache` param.
- Link to **Key Concepts** for description of Buck directory structure:
  projects, packages, cells.

Reviewed By: styurin

fbshipit-source-id: 28b2b9f20c

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