Polish the documentation for Mix.Task.alias?/1
Polish the documentation for Mix.Task.alias?/1

I added a link to the documentation for the Mix module, in particular to
the section that talks about aliasing.

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