Small improvements to repo analysis docs (#4413)

* Reflows the text so it's all sentence-per-line.

* Improves the one-line summary at the top of the article.

* Explains why you must run increasingly large analyses rather than just
  accepting the first run.

* Expands the docs about why failures are bad even if snapshots appear
  to work.

* Mentions verifying the reference implementation before reporting ES
  bugs.

Author: DaveCTurner