Expand the explanation of how to go and do useful work in non-blocking

Expand the explanation of how to go and do useful work in non-blocking

Add additional commentary to the non-blocking examples explaining where to
add code to go and do other useful work.

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Hugo Landau <hlandau@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/21950)