Provide better documentation for lint engines, linters, and the PyLint linter

Summary:
Document the relationship between lint engines and linters. Provide an example
linter. Improve the documentation of PyLintLinter, which has a bunch of
configuration stuff which you had to dig into the code to get.

Test Plan:
Ran "arc lint --engine ExampleLintEngine --lintall derp.py" on a file with a
Python syntax error in it. Read documentation.

Reviewed By: j3kuntz
Reviewers: j3kuntz, andrewjcg
CC: aran, j3kuntz
Differential Revision: 557