Naming inconsistencies in Enum.group_by/3 (#5031)
Naming inconsistencies in Enum.group_by/3 (#5031)

Renamed `mapper_fun` to `value_fun`, to match docs and other function definitions.

Renamed `fun` to `key_fun` in docs, to match doctest and function definitions. It was previously not completely clear if `fun` referred to `key_fun` or `value_fun`.

gfvcastro/elixir,lexmag/elixir,elixir-lang/elixir,lexmag/elixir,kimshrier/elixir,antipax/elixir,ggcampinho/elixir,michalmuskala/elixir,antipax/elixir,gfvcastro/elixir,joshprice/elixir,pedrosnk/elixir,kelvinst/elixir,pedrosnk/elixir,beedub/elixir,ggcampinho/elixir,kelvinst/elixir,beedub/elixir,kimshrier/elixir