docs(guide/Directives): remove confusing definition object message

[jdai8]

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

Docs update

Please check if the PR fulfills these requirements

• [x ] The commit message follows our guidelines: https://github.com/angular/angular.js/blob/master/CONTRIBUTING.md#commit-message-format
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

Other information:

Prior to this point, the doc does not mention returning a postLink function, and all the examples use the definition object form. New readers could misinterpret "returning a function" as returning the factory function mentioned in the previous section.

Since this page is only a "gentle introduction" to directives, using the definition object is a best practice, and the message is already echoed in the docs for $compile, I think we should just remove the message altogether.

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[jdai8]

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