docs(guide/Directives): mentioned shorthand for "return function()"

[zbycz]

The shorthand is little confusing, added clarification.

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[Narretz]

Hi, thanks for your PR! However, before we can merge we need you to sign the CLA.

[Narretz]

Imo, we should change the example to use the verbose way of defining a directive. That makes it clearer.

[lgalfaso]

Agree with @Narretz

[zbycz]

Quite contrary, i'd like to learn that the shorthand exists - it can be seen in other projects as well. Just the clarification was missing.

[googlebot]

CLAs look good, thanks!