transition docs/tests/messages to sha256 :no_check

`sha256 :no_check` has been supported syntax for a couple of months,
and now is being transitioned to the preferred syntax, replacing
the `no_checksum` stanza (which will still be supported, but
deprecated)
Merge pull request #4257 from rolandwalker/convert_no_checksum_1

transition docs/tests/messages to sha256 :no_check