This commit changes the behavior of the builtin SSHRun action to use a Windows
shell if the WinSSH communicator is active. This allows for running one-off SSH
commands with Windows Command Prompt or PowerShell. By default, this will not
allocate a TTY for any SSH commands.
Example usage:
```
vagrant ssh -c 'dir "c:\program files"'
```
Updates docs to reflect a change made in
a55a53e6a46438d5093487f83248f01ddece4534.
7fb7dd8608