docs(ngShowHide): Added note about falsy values in ngShow and ngHide

I've been bitten by this several times, and it's not documented anywhere. Since `angular.toBoolean` considers additional values as false, it should be documented somewhere. This could probably go into a separate page which allows easier maintainability if/when the values change.

Other usages include:
 - `orderBy`
 - `ngIf`
 - `input`

Should I include documentation for these as well? What do you think?