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?
Wasn't aware of it and quite surprised me :)
I had an issue that user was trying to set `Ctrl` shortcuts in Git extension but it got binded to `Cmd` instead, so I had to look through the codes to realize I have to specify `brackets.platform` as a third argument for `addBinding` method.