Added documentation for /manage/cmdshell.py

No code changes, just documentation. I tested the :class:`paramiko.channel.Channel` links with a documentation build. They generate bold text, but are not clickable URLs since they wouldn't resolve anyway. Is that an acceptable solution, or should I refer to the paramiko objects with different mark up?

I think I got everything right. Let me know if I messed up anywhere or if any of my descriptions are inaccurate and I'll change it.

I made these same changes to my master branch. Do I need to create a pull request for that as well so the current version gets updated?

On an side note, I'm wondering why the isdir() function in the SSHClient class returns 1 and 0 instead of True and False. The equivalent function in the LocalClient class returns True and False. Just curious.