docs(guide/directive): remove confusing best practice

Prior to this point, the docs do not mention returning a postLink function, and all the examples use
the definition object form. So, this message is confusing to new readers who may misinterpret
"returning a function" as returning the factory function mentioned in the previous section.

Since this page is only a "gentle introduction" to directives, and using the definition object is a
best practice, it is best to just remove the message altogether.

Closes #14871