docs(angular.isArray): Document that isArray is an alias

[graingert]

What kind of change does this PR introduce? docs update

Does this PR introduce a breaking change?
no

Please check if the PR fulfills these requirements

• The commit message follows our guidelines: https://github.com/angular/angular.js/blob/master/CONTRIBUTING.md#commit-message-format

[Narretz]

This is technically an implementation detail, why do you want to document this?

[graingert]

@Narretz so users can make an informed decision about whether to use Array.isArray or angular.isArray

[gkalpak]

_{For the record, I agree this is an implementation detail (was different in the past and may change in the future - not likely but possible). We don't document other helper/function implementations to allow informed decisions. Just saying...}