Added description to most settings.

I thought adding short quotes (from the [Ruby Style Guide](https://github.com/bbatsov/ruby-style-guide)) to the `.rubocop.yml` file describing what the setting does would be useful. I didn't manage to comment on the settings: `IfWithSemicolon`, `MultilineIfThen`, and `OneLineConditional` - these can be however completed by the author (or someone else).

Notably, some original comments were shortened or broken down into individual parts though their context remains the same. As for tests, I've ran the Rakefile and that's the output I got:

```
Finished in 15.79 seconds
196 examples, 0 failures
```

*The changes made are after all just plain comments...*