Improve documentation about configuration and linter options

[akaihola]

Thank you for your help! It works properly. Possible improvements in docs:

• --help command can specify that config option is only for black/isort/darker things
• --help command can specify that -L option config is taken automatically from root directory as normally and you can pass arguments as normally
• Maybe you can also mention these things in your GitHub page docs, I was really confused that black settings work properly with --config option but pylint doesn't

Originally posted by @simonf-dev in #398 (reply in thread)