Add some docs to `Kernel.defprotocol/2`
Add some docs to `Kernel.defprotocol/2`

I added a paragraph mentioning that the `:for` option passed to `defimpl` can be
omitted when the implementation is inside the module for which the protocol is
being implemented. This wasn't mentioned anywhere.

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