Update expression.ngdoc

Please, update the section on one-time bindings by another example showing how the feature can be used with filter (chains). I was very surprised - positively - when I discovered that feature today and wished I could have read something about it before, which would have saved me some time.
Well, at least for the tests that I did using 1.3.0-beta18, the two versions behaved differently in the way I have described it. More precisely, the two expressions that I have tested were `{{ ::'myString' | myFilter }}` and `{{ ::('myString' | myFilter) }} where `myFilter` is a filter that we have developed, which is closed sourced and therefore I cannot release its source here. To say little more about the filter, it's quasi an asynchronous filter that it depends on a service that in turn needs to fetch a resource asynchronously first in order to be able to work meaningfully. The filter returns `undefined` unless it is ready and irrespective of the input. So, while `{{ ::'myString' | myFilter }}` got stable immediately (seemingly because of the constant string expression) and in our case always before the filter was ready (i.e., while it was still returning `undefined`), the other version `{{ ::('myString' | myFilter) }} got stable always after the filter was ready.

I haven't further analyzed Angular's source code, but it seems that this is a feature implied by the code.