Update 'addBinding' documentation in KeyBindingManager.

Related to issue https://github.com/zaggino/brackets-git/issues/846
Btw what is the reason for this key replacement?
> Btw what is the reason for this key replacement?

It's to make specifying shortcuts simpler. Usually a command with a shortcut of `Ctrl` on Windows is `Cmd` on Mac.

Is this causing a problem, or you just weren't aware of it?