Tidy up buck documentation

- Enable source highlighting for `maven_jar()` example.

- Break the `buckd` documentation into paragraphs for easier
  reading, and fix a couple of grammatical mistakes.

- Add indentation and remove "$>" on `buckd` usage example to
  make it consistent with other examples.

- Add missing backticks.

Change-Id: Ia65dec84969c38612f449309a9a29e648d60f828