Improve the documentation of the config module.

  * Document how to load configurations from file / cli arguments;
  * Deduce None to Opt instead of StrOpt; no visible change.

Change-Id: I9a6b5e02d7ea75ce23fddc36cff85644d2b5f874
Merge "Improve the documentation of the config module."
Message support in WSGI.

  * API limits (on TTL, e.g.) are not verified (noted in code).

Implements: blueprint transport-wsgi

Change-Id: I8c3908ce7e74da26b3ebebd06f308754665d681b