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?
I should have followed the link first :)

Shortcuts specified for keybindings are cross-platform, so `Ctrl` is translated to `Cmd` on Mac for reason stated [here](https://github.com/adobe/brackets/pull/10225#discussion_r22081986).

But, user key map is user-specific, which is more likely to be machine-specific, so we do not translate `Ctrl` is translated to `Cmd` on Mac in this case.