Improve wording in nova-api-test-inheritance

This commit improves the wording of the last section of the proposed
change. It was a little unclear that it was outline the case where
the inheritance wasn't possible between the v3 and v2 tests, because
there is insufficient overlap between the 2 versions. This just
changes the wording slightly to highlight that and make it a little
clearer.

Change-Id: I29b2084b4282499758c03784d339a4dbb5680d67
Merge "Improve wording in nova-api-test-inheritance"