Add section titles and fix markup of docstring

Add section titles to the module docstring for cfg.py so
http://docs.openstack.org/developer/oslo.config/cfg.html is broken up
and easier to scan quickly.

Fix a couple of minor markup issues at the same time.

Change-Id: I03df6842b29e62de1b3c923218f174cf65fdefdb
Merge "Add section titles and fix markup of docstring"