Elixir: Standardize function expressions with multiple arities `fun/1,2` in the documentation

[fertapric]

As discussed in the Elixir mailing list.

cc @hassanshaikley

[hoominkani]

@fertapric
May I try it?
This is the first time for me to contribute to open source.
I will convert function expressions with multiple arities like fun/1/2 to fun/1,2 in the documentation.

[josevalim]

I will convert function expressions with multiple arities like fun/1/2 to fun/1,2 in the documentation.

Perfect. Please go ahead and let us know if you have any questions.