scrooge: Add section on programmatic codecs and metadata usage
scrooge: Add section on programmatic codecs and metadata usage

Problem
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Developers don't know about the APIs that let them write generic
Scrooge usage without reflection.

Solution
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Write a synopsis about these APIs in the Scrooge User Guide.

JIRA Issues: CSL-11888

Differential Revision: https://phabricator.twitter.biz/D892126

twitter/scrooge,twitter/scrooge,twitter/scrooge,twitter/scrooge