Add "usecase" changelog tag

Add "usecase" changelog tag

"usecase" indicates the library now supports something a user
was trying to do.  It's not quite a "feature" since it's something
that seems like it should have worked, it's not a "bug" because
no mistake was made, it's just something that wasn't considered before.
The advantage of "usecase" is that it inherently suggests a different
style of prioritization vs. something that is preventing the library
from working as designed.

This change also adds docs/build/conf.py under the pep8 formatting
test coverage.

Change-Id: I790ccb799864fec48e4695aedeed2814ab9d493e