comments and docs for regression and tracking info

[clmould]

Description

Closes #3744
Added some comments to the yml file, and a bit more clarity in the regression section of the docs to explain the reference mfiles are updated upon merge to main

Checklist

I confirm that I have completed the following checks:

• My changes follow the PROCESS style guide
• I have justified any large differences in the regression tests caused by this pull request in the comments.
• I have added new tests where appropriate for the changes I have made.
• If I have had to change any existing unit or integration tests, I have justified this change in the pull request comments.
• If I have made documentation changes, I have checked they render correctly.
• I have added documentation for my change, if appropriate.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 48.04%. Comparing base (09c52f5) to head (895c128).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4162   +/-   ##
=======================================
  Coverage   48.04%   48.04%           
=======================================
  Files         144      144           
  Lines       30211    30211           
=======================================
  Hits        14515    14515           
  Misses      15696    15696           

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