Reinstate ruff rule D404

[john-science]

We are currently ignoring ruff rule D404:

https://github.com/terrapower/armi/blob/477d132e901f65d1b778b958ba61cb291d42bdf8/pyproject.toml#L187

We are not doing this for a good reason, it will just take a minute to fix it. Essentially, docstrings shouldn't start with the word This, because that is obvious and implied. It's just bad writing. There are, unfortunately, 106 such docstrings in ARMI. I would love it the pyproject.toml file was changed and these docstrings were fixed.