Update Documentation MRC -> Frequency

[wilwade]

Our Design Docs and Code documentation use the old MRC a lot. We need to update them to all use Frequency

• Should also work with the authors of any design docs prs to update those without conflicts. Specifically Designdoc/batching #90
• Should NOT update any code paths or code variable names. That is MRC -> Frequency Code Updates #212
• Careful with find and replace!