bpo-38056: overhaul Error Handlers section in codecs documentation (#15732)

* Some handlers were wrongly described as text-encoding only, but actually they can also be used in text-decoding.
* Add more description to each handler.
* Add two REPL examples.
* Add indexes for Error Handler's name.

Co-authored-by: Kyle Stanley <aeros167@gmail.com>
Co-authored-by: Victor Stinner <vstinner@python.org>
Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>