Fix IEx doc printing for headings with newline characters (#13018)

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.

lib/elixir/lib/io/ansi/docs.ex

@@ -50,6 +50,10 @@ defmodule IO.ANSI.Docs do
   """
   @spec print_headings([String.t()], keyword) :: :ok
   def print_headings(headings, options \\ []) do
+    # It's possible for some of the headings to contain newline characters (`\n`), so in order to prevent it from
+    # breaking the output from `print_headings/2`, as `print_headings/2` tries to pad the whole heading, we first split
+    # any heading containgin newline characters into multiple headings, that way each one is padded on its own.
+    headings = Enum.flat_map(headings, fn heading -> String.split(heading, "\n") end)
     options = Keyword.merge(default_options(), options)
     newline_after_block(options)
     width = options[:width]

lib/elixir/test/elixir/io/ansi/docs_test.exs

@@ -33,6 +33,14 @@ defmodule IO.ANSI.DocsTest do
       assert String.contains?(result, " foo ")
       assert String.contains?(result, " bar ")
     end
+
+    test "is correctly formatted when newline character is present" do
+      result = format_headings(["foo\nbar"])
+      assert :binary.matches(result, "\e[0m\n\e[7m\e[33m") |> length == 2
+      assert ["\e[0m", foo_line, bar_line, "\e[0m"] = String.split(result, "\n")
+      assert Regex.match?(~r/\e\[7m\e\[33m +foo +\e\[0m/, foo_line)
+      assert Regex.match?(~r/\e\[7m\e\[33m +bar +\e\[0m/, bar_line)
+    end
   end
 
   describe "metadata" do