Update `DateTime.from_unix` documentation (#9840)
Update `DateTime.from_unix` documentation (#9840)

The documentation stated that negative numbers down to -62167219200 are
supported, but it actually works down to -377705116800. Add doctests to
illustrate the upper and lower boundaries.
pedrosnk/elixir,ggcampinho/elixir,kelvinst/elixir,elixir-lang/elixir,kimshrier/elixir,kelvinst/elixir,pedrosnk/elixir,ggcampinho/elixir,michalmuskala/elixir,kimshrier/elixir,joshprice/elixir