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.
This isn't quite right. `::(text | someFilter)` is actually the same as `::text | someFilter`.
We consider an expression to be one-time binded when the first two non-whitespace characters are "::", regardless of what follows.

The first half of the added diff is correct though and worth elaborating in the doc (the avoid re-filtering part).