Update docs to indicate that IPv6 is supported with the nft method.

- Adds nft into the requirements.rst file.

- Update description of what happens when a hostname is used in a
  subnet.

- Add ipfw to list of methods.

- Indicate that --auto-nets does not work with IPv6. Previously this
  was only mentioned in tproxy.rst

- Clarify that we try to use "python3" on the server before trying
  "python".