This cleans up the docs for upgrading Review Board and site directories.

Most of these changes are pretty minimal, involving:

Moving to code-block:: console.
Providing some additional details on needing to be root.
Removing the step for restarting memcached (as we're better at key

  management these days).
Moving to itemized lists of steps.
Including the SELinux steps during site upgrade.

The "Troubleshooting Upgrades" section has been removed, as it's no

longer applicable. There is a blurb at the right upgrade step to give

users direction if upgrades fail, though.

Built the docs and read through them. Checked for spelling errors,

bad links, and build errors.