Updates to documentation and examples.

[fireproofsocks]

Adds an example of schema definition that uses additional options and provides a cross reference to the field macro.

This PR addresses issue #678
No code was changed

I attempted to run all tests, but I am encountering this issue with my setup that I cannot figure out:

mix test
==> fs (compile)
Compiling c_src/mac/cli.c
c_src/mac/cli.c:1:10: fatal error: 'getopt.h' file not found
#include <getopt.h>
         ^~~~~~~~~~
1 error generated.
ERROR: compile failed while processing /Users/everettgriffiths/DropboxDM/absinthe/deps/fs: rebar_abort
** (Mix) Could not compile dependency :fs, "/path/to/.mix/rebar compile skip_deps=true deps_dir="/path/to/absinthe/_build/test/lib"" command failed. You can recompile this dependency with "mix deps.compile fs", update it with "mix deps.update fs" or clean it with "mix deps.clean fs"

So unfortunately, I cannot run the tests, so even though this PR only updates documentation and comments, I have to disclose that fact.

[benwilson512]

Thank you!