Document testing guide for new API contributions

[OpenStack Infra]

https://review.openstack.org/529618
Dear bug triager. This bug was created since a commit was marked with DOCIMPACT.
Your project "openstack/nova" is set up so that we directly report the documentation bugs against it. If this needs changing, the docimpact-group option needs to be added for the project. You can ask the OpenStack infra team (#openstack-infra on freenode) for help if you need to.

commit 749f1ecbc5f3bf0a499bec328b158bf2b1d69240
Author: Matt Riedemann <email address hidden>
Date: Thu Dec 21 10:22:49 2017 -0500

    Document testing guide for new API contributions

    This fills in the TODOs for the unit, functional and
    docs part of the API contributor guide.

    Since we don't rely on the DocImpact tag in the commit
    message for API changes (that tag results in a nova bug
    and was meant mostly for making changes to docs external
    to the nova repo, which is no longer the case), this
    changes that section to just talk about the in-tree docs
    that should be updated for API changes.

    Change-Id: I9ca423c09185d2e3733357fd47aaba82d716eea4