If the same parameter is used twice in --config, last one wins
If the same parameter is used twice in --config, last one wins

Summary:
- If the same configuration option is used more than once with the `--config` parameter, the last values specified prevail.
- The `--flagfile` parameter is a synonym for `@`.
- Start the description for each parameter as a new paragraph.
- Other minor edits.

Reviewed By: philipjameson, ttsugriy

fbshipit-source-id: 699693e

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