Document what exception impl_for!/1 raises (#10619)
Document what exception impl_for!/1 raises (#10619)

Related: https://github.com/elixir-lang/elixir/pull/10617
joshprice/elixir,kimshrier/elixir,pedrosnk/elixir,ggcampinho/elixir,ggcampinho/elixir,kelvinst/elixir,kimshrier/elixir,elixir-lang/elixir,kelvinst/elixir,michalmuskala/elixir,pedrosnk/elixir