Improve settings documentation

I was reading the documentation to configure another application successfully deployed with Gunicorn, and thought the documentation here could be improved a bit for clarity.

* Use code highlighting for the names, to indicate they are the names to use in the Python configuration file.
* Spell out the CLI flags and defaults with "Command line" and "Default" prefixes so it's clear what these are
* Consistently use Python syntax for the defaults, so the types may be more easily understood.
* Split multiple CLI flags with "or" rather than a comma, for clarity and easier copy-paste.
* Improve some descriptions, such as for ``raw_env`` and a header for "Server Hooks".