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.
Being familiar with Angular's source code, I can assure you that both are evaluated the same by the expressions parser. If they're behaving differently in your app, it's probably something unrelated that's causing that.

Here's a plnkr trying to replicate what you describe, a constant value passed to an _asynchronous filter_ with one-time binding in the expression: http://plnkr.co/edit/mHmuSTu0KR95YZHzZ5Lf?p=preview

If I missed anything, please let me know. If you can link to a plnkr that reproduces your scenario, that would be great.

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Again, I believe the first part of the doc changes are worth adding, and maybe even elaborating on