Fix and improve doctest's documentation

[serhiy-storchaka]

When working on #108885 I found several issues in the doctest documentation.

• doctets.failureException does not exist. The referred exception is unittest.TestCase.failureException.
• Several copy-paste and markup errors.
• Example for float formatting is outdated since Python 3.0. repr() and str() for floats now have the same precision and do not depend on the platform C library.

Linked PRs

• gh-134918: Fix and improve doctest's documentation #134919
• [3.14] gh-134918: Fix and improve doctest's documentation (GH-134919) #134966
• [3.13] gh-134918: Fix and improve doctest's documentation (GH-134919) #134967