Add function to apply fill-region in doc strings

[rranelli]

This commit adds a function that helps one to keep documentation strings
under fill-column columns.

For example, consider the following doc strings:

  @moduledoc """
  This is module documentation with a line that is waaaaayyy to long to be read. It's pretty bad when lines get wrapped in your editor or when you have to scroll horizontally to be able to see it well. Gosh, can Emacs relieve me of the pain of keeping documentation columns straight??
  """

Github adds horizontal scrolling to the lines above, which I find a real pain.

  @doc """
  This is module documentation with a line that is waaaaayyy to long to be read. It's pretty bad when lines get wrapped in your editor or when you have to scroll horizontally to be able to see it well. Gosh, can Emacs relieve me of the pain of keeping documentation columns straight??
  """

After calling elixir-mode-fill-doc-string the result would be:

  @moduledoc """
  This is module documentation with a line that is waaaaayyy to long to be read.
  It's pretty bad when lines get wrapped in your editor or when you have to
  scroll horizontally to be able to see it well. Gosh, can Emacs relieve me of
  the pain of keeping documentation columns straight??
  """

  @doc """
  This is module documentation with a line that is waaaaayyy to long to be read.
  It's pretty bad when lines get wrapped in your editor or when you have to
  scroll horizontally to be able to see it well. Gosh, can Emacs relieve me of
  the pain of keeping documentation columns straight??
  """

[whatyouhide]

👍 👍 👍 wonderful feature!

[rranelli]

I did not understand why the CI build failed, the output of the failed job was quite clobbered.

I also could not reproduce it locally.

[mattdeboard]

I will try to replicate this failure locally.