Update `IEx.introspection.print_doc/5` to trasverse the list of headings
and split each heading into multiple headings, in case the heading has
any newline characters (`\n`). This fixes an issue where using the `h`
macro with some functions ended up with a broken output for the function
signature, as the padding done by `IO.ANSI.Docs.print_headings/2` does
not take into consideration the fact the the headings can contain
newline characters, and thus end up in multiple lines.
Dino
d332f262e7