Enhancing logstash docs for adding new files

Adding some extra verbiage to the docs to better explain how
the message tags tie in to telling logstash how to parse things.
Also noting which file to edit when adding new files, how to add
new parsing patterns, and some troubleshooting tips.

Change-Id: Ie91effd4b3ff3627ec040eb5c54b972e82add0e7
Merge "Enhancing logstash docs for adding new files"