Minor processor quota computation fix + docs

[arunramani]

Description

Set processor quota to -1 if quota is negative or period is 0. Added docs on how to enable the monitors

This PR has:

• been self-reviewed.
• added documentation for new or modified features or behaviors.
• added Javadocs for most classes and all non-trivial methods. Linked related entities via Javadoc links.
• added comments explaining the "why" and the intent of the code wherever would not be obvious for an unfamiliar reader.
• added unit tests or modified existing tests to cover new code paths, ensuring the threshold for code coverage is met.
• been tested in a test Druid cluster.

[abhishekagarwal87]

LGTM

[techdocsmith]

Minor style suggestions. Otherwise LGTM. I'm figuring that anyone looking to deal with monitoring at this level will understand standard abbreviations.