We probably want to introduce multiple explain subcommands and
overloading `explain` to explain it all seems like a bad idea.
We may want to introduce a subcommand to explain config options and
config options may end up having the same name as their rules, e.g. the
current `banned-api` is both a rule name (although not yet exposed to
the user) and a config option.
The idea is:
* `ruff rule` lists all rules supported by ruff
* `ruff rule <code>` explains a specific rule
* `ruff linter` lists all linters supported by ruff
* `ruff linter <name>` lists all rules/options supported by a specific linter
(After this commit only the 2nd case is implemented.)
dd79ec293a