Update docs for `File.dir?/1` (#5581)
Update docs for `File.dir?/1` (#5581)

The documentation for the `File.dir?/1` function didn't explicitly
state that certain shell conventions (like `~`) would not be
automatically expanded in this function, so I'm adding more
clear documentation including examples for how to use this function.
pedrosnk/elixir,kimshrier/elixir,kelvinst/elixir,gfvcastro/elixir,beedub/elixir,pedrosnk/elixir,lexmag/elixir,ggcampinho/elixir,beedub/elixir,gfvcastro/elixir,antipax/elixir,antipax/elixir,kelvinst/elixir,lexmag/elixir,elixir-lang/elixir,ggcampinho/elixir,joshprice/elixir,michalmuskala/elixir,kimshrier/elixir