Fix docs that use safe_to_string

[brianberlin]

any ideas on how to format the doctests that use safe_to_string better?

Eg. https://hexdocs.pm/harmonium/Harmonium.html#number_input_stack/3

[grossvogel]

We could definitely go from \\\" and \\n to \" and \n by using ~S on the doc blocks, but I don't think there's a way to put the linebreaks in there instead of \n.

On some of the tests that exercise specific options we might get some mileage by showing that a certain parameter causes a certain substring to appear in the output rather than checking the whole output, but that's not going to work in general, and it takes away some of the illustrative power of the examples.

It might be that the best way to do this would be to let the docs focus on teaching the functionality and move the ugly test cases into regular test modules (where they can use heredoc and therefor be much more tractable)