R1:2e0301d647ae

Update repository hosting documentation for all the issues users have hit

Summary:
Ref T4151. Addresses these issues:

- Mentions `diffusion.ssh-user`.
- Mentions `/etc/shadow` and `!!`.
- Mentions `/etc/passwd` and shell.
- Mentions `sshd -d -d -d`.
- Mentions `Defaults requiretty`.
- Adds `AllowUsers` to default configuration.
- Mentions `sudo -E ...` as a troubleshooting step.
- Mentions multiple VCS binaries.
- Fixes `sshd` paths to be absolute.
- Fixes example path in…