Comment 4 for bug 1822000

Reviewed: https://review.openstack.org/648294
Committed: https://git.openstack.org/cgit/openstack/cinder/commit/?id=09c1111904410c22d16b850fa142e9e12f8c33c9
Submitter: Zuul
Branch: master

commit 09c1111904410c22d16b850fa142e9e12f8c33c9
Author: Brian Rosmaita <email address hidden>
Date: Wed Mar 27 18:46:10 2019 -0400

    Document behavior of message.create

    Add docstrings to clarify the behavior of message.create() when both
    an exception and detail are passed as arguments. Updates the
    user_messages section of the contributor docs to reflect the changes
    introduced in Pike by implementation of the "Explicit user messages"
    spec. Adds tests to prevent regressions.

    Closes-bug: #1822000
    Change-Id: Ia0425dc9c241d0c101057fbc534e68e7e5fdc317