Enhance API documentation

[paulo-ferraz-oliveira]

Document the API a bit better.

First, discuss what the API actually is.

Then, spec it as best as possible.

Then, write ex_doc for it.

Then, hide (from doc.) what's not public.

[paulo-ferraz-oliveira]

Here's a starter...

• elvis_core

  • elvis_core:rock/1

• elvis_config

  • elvis_config:config/0
  • elvis_config:default/0
  • elvis_config:from_file/1
  • elvis_config:resolve_files/2
  • 5 x elvis_config:set_.../1
  • type elvis_config:t()

• elvis_result

  • type elvis_result:item()
    • should be renamed to elvis_result:t()?

• elvis_rule

  • type elvis_rule:t()

• elvis_file

  • elvis_file:find_files/2
    • should be moved out of the API? (easy to replicate)