Documented Stevedore usage and source details

Updated architecture document to mention Stevedore
and make a correct statement about polling frequency.
Updated configuration document to reference the relevant
section of the architecture document.
Documented the name parameter of the source stanzas in pipeline.yaml.
Documented the behavior when multiple source stanzas match one plugin.

Change-Id: I0d09f2bd8943425cd250574df41e77a68bdcf55e
Merge "Documented Stevedore usage and source details"