Docs: Update To List Custom Analyzers
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### Problem to solve
* What docs or doc section affected? Include links or paths.
* https://docs.gitlab.com/12.1/ee/user/application_security/dependency_scanning/analyzers.html
* Is there a problem with a specific document, or a feature/process that's not addressed sufficiently in docs?
* Shows ability to customize analyzers, but does not list them.
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