Update map.ex (#6267)
Update map.ex (#6267)

Clarify documentation on Map.update/4.
michalmuskala/elixir,kelvinst/elixir,kimshrier/elixir,lexmag/elixir,lexmag/elixir,pedrosnk/elixir,antipax/elixir,kimshrier/elixir,beedub/elixir,beedub/elixir,ggcampinho/elixir,pedrosnk/elixir,antipax/elixir,gfvcastro/elixir,ggcampinho/elixir,elixir-lang/elixir,joshprice/elixir,gfvcastro/elixir,kelvinst/elixir