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

[svetlyak40wt]

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

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.