fix: Re-clarify unit test REVIEWING documentation
fix: Re-clarify unit test REVIEWING documentation

This patch set corrects some misleading documentation under "Unit
Tests" section in REVIEWING.rst.

It currently claims that service clients do not require unit test
coverage -- but this is false. This is because Tempest now places
all of its service clients in tempest.lib. And as per
https://docs.openstack.org/tempest/latest/library.html#testing it
is required to add unit tests for all service client interfaces.

Thus this makes the documentation language clear that service clients
require unit tests.

Change-Id: Iac6ba71496e20b6724a6a2ca4ec2beb42f2a58c8

cisco-openstack/tempest,openstack/tempest,openstack/tempest,masayukig/tempest,cisco-openstack/tempest,masayukig/tempest