documenting the default return value for `Module.get_attribute/2`
documenting the default return value for `Module.get_attribute/2`

refactor the docs so the first line is short

Went ahead and did the whole module.

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