Doc enhancement for API service deployment with mod_wsgi

Now Pecan is sharing the same debug configration with Ceilometer,
but DebugMiddleware of Pecan doesn't support multi processes.
That means user can't enable the Ceilometer debug mode if he is
using multi process mode for mod_wsgi config.

Closes-Bug: 1352088

Change-Id: I6e83fbcbee23a519fe26c3036f42cc619848a989
Merge "Doc enhancement for API service deployment with mod_wsgi"