Improve default content function

[GaryJones]

While expand_content() can be dropped by developers in favour of a custom output, the default function should still be improved.

It is currently very focused towards how functions look, and not classes and hooks. For instance, the prototype doesn't make a lot of sense of classes or hooks.

The Arguments heading is output regardless of whether there are any argument or not.

The function name itself is also a little too generic - wpfuncref_content() might be better for instance.

[Rarst]

This was refactored out into WP_Parser\the_content() template tag, filtering approach was just too full of a trouble.

I hadn't looked much into what it does though. DevHub theme currently doesn't use these bits actually.