Someday the entire Pelican code base will perhaps have Black and isort
formatting applied to it, but it doesn't make sense for the linter to
check that until it's actually done. This changes the `lint` Invoke task
to run Flake8 on changed lines only, which should bring it into line
with both the current Pre-commit and CI configurations.

Also, the docs erroneously stated that `invoke tests` would also check
for code style compliance, which has herein been fixed.

Document and clarify that only Pip-installed namespace plugins will be
shown when running the `pelican-plugins` command.