Adde documentation/example for header_checks

[Dzeri96]

While trying to use postfix_header_checks, I had to go into the template in order to figure out what the shape of the object in the array should be. I think we should document this shape, and any other complex ones like in postfix_aliases. Finally, an example could be shown since it's not quite clear if the / symbol should be included in the pattern.

I could do this with a PR, but first let me know how you'd express the shape of these objects and which ones are to be documented.