Update directive.ngdoc

[bradw2k]

More description of "&" bindings, the fact that they trigger evaluation of expressions in the original scope's context.

[mary-poppins]

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[caitp]

Looks okay to me

[bradw2k]

"& bindings expose a function to an isolated scope" is misleading, & bindings expose expressions to the isolate scope. Expressions which do not call functions are perfectly valid and useful in & bindings.