Overhaul of dmarc.md

[TonyGravagno]

This ticket documents a series of updates that are being applied to dmarc.md.
As usual there are a number of trivial cleanup details and content clarifications. But the current doc is not adequate to guide a user/admin smoothly through this area. Significant changes are required to bring the material up to date, and to guide a user/admin smoothly through configuration. This is especially true now with code changes recently made in v3.3.

Examples of proposed content revisions

• The introduction on this page needs to clarify exactly what this module does.
• Clarification is required for configuration of each feature of this module, including determination of disposition, actions, Redis logging, and outbound reporting of aggregate data to domain owners.
• Clarifications and references are required regarding the Redis details - for using the common redis configs or for using Redis configs that are specific to DMARC processing.
• Details have changed and enhancements have been made in v3 related to exclusion of eSLDs for sampling, and also for reporting
• The munging section is recent, adequate, and only requires trivial tweaks.

( Many of these changes are already complete and in-progress. )

My intent for this ticket is that I will offer a series of small PRs to get the content current and complete, without striving for ideal. Subsequent tickets can move forward with refinements and enhancements. I understand the material and am qualified to do all of the work described here. But there are nuances that require confirmations and corrections from @vstakhov, and with respect for his time and to ensure quality, my intent is to move through this slowly.

I'm hoping @fatalbanana, @moisseev, and others will offer to help vet this material and comment. As noted in #502, this might be a good project to get other eyes on the process, get suggestions for content, and get other participation from the user base. A Discussion section in this repo would help, so that tickets like this for specific actions can be separated from random comments and suggestions.

Thanks for your time.