Alter the documentation on Ember.Binding Class

The way the manual binding section is written now confused the heck out of me and really led me on a wild goose chase.

As it stands, the documentation seems to suggest that `from` receives the actual object.property you want to bind and `to` receives a path that describes the location of a property. As far as I can tell, both `from` and `to` must actually be paths to the value relative to whatever is passed into `connect`.

This PR alters the documentation as such.

It also adds a brief example illustrating that it is possible to bind properties across objects manually.