Improve documentation around jurigged monkeypatch and add tests that prove value

[JamesHutchison]

The jurigged monkeypatch is insufficiently documented. It does two functions:

• Applies the assertion rewrites from pytest when a function is updated
• Fixes an off-by-one issue

The PR that added the monkeypatch and assertion rewrites came first and is here: https://github.com/JamesHutchison/pytest-hot-reloading/pull/15

The PR that fixes the off by one is here: https://github.com/JamesHutchison/pytest-hot-reloading/pull/32

I'd need to double check, but IIRC the off-by-one was somewhat prevalent. When the line numbers are wrong, the varname library used by MegaPatch.it can't find things (due to looking in the wrong spot) and those break. Additionally, the wrong line numbers can break debugger behavior because things stop lining up.

[JamesHutchison]

It's also been many months, it might be time to add some issues to the jurigged library.

[JamesHutchison]

Comments were improved in #55

I'm wondering if it makes more sense to just get this all fixed in jurigged instead of writing tests for the monkeypatches