Document cinder API service multi-process config

This patch adds a section on how to use the new osapi_volume_workers
setting to increase the throughput of the Block Storage API service.

Partially implements: https://blueprints.launchpad.net/cinder/+spec/multi-process-api-service

Change-Id: I99f2d544bfccfe15c57f85b47c0da1b09ac54df6
Merge "Document cinder API service multi-process config"