Update documentation because assert-that does not produce nice strings under the Rove

Previously, when used under Prove, `assert-that` call produced a nice output:

```lisp
TEST> (assert-that task
(has-slots 'title "The title "))
✓ Has slots:
TITLE = "The title "
```

However, for Rove, it just returns `t` or `nil`.

We need to create a helper for a demonstration of a matcher without a call to `assert-that`. And to rewrite all docstrings to use this helper.

This way, all examples in the documentation will become easily reproducible.