Merge branch 'am/doc-sha256'

Tone down the warning on SHA-256 repositories being an experimental
curiosity.  We do not have support for them to interoperate with
traditional SHA-1 repositories, but at this point, we do not plan
to make breaking changes to SHA-256 repositories and there is no
longer need for such a strongly phrased warning.

* am/doc-sha256:
  doc: sha256 is no longer experimental