docs(guide/expression): fix docs re $window

The documentation on context is incorrect and misleading:

1. "Angular expressions must use $window explicitly to refer to the global
   `window` object": expressions cannot access `$window`
1. The example doesn't actually attempt to use $window in a expression. It's in a
   function called from an expression, which incorrectly implies to readers that:
  1. functions ARE expressions
  1. functions called by expressions can't access `window`

Here's [a plunkr](http://plnkr.co/edit/Gd4xAV?p=preview) to make both these issues clear.

This change fixes the errors and informs the reader about Angular's `$window` etc services,
and adds an explicit example of an expression not being able to access `window`.