While ruff displays the string returned by Violation::message in its
output for detected violations the messages displayed in the README
and in the `--explain <code>` output previously used the
DiagnosticKind::summary() function which for some verbose messages
provided shorter descriptions.
This commit removes DiagnosticKind::summary, and moves the more
extensive documentation into doc comments ... these are not displayed
yet to the user but doing that is very much planned.
8993baab01