add interfaces docs + module interfaces

[glozow]

• Targeted PR against correct branch (see CONTRIBUTING.md)

• Linked to github-issue with discussion and accepted design OR link to spec that describes this work.

• Wrote tests

• Updated relevant documentation (docs/)

• Added a relevant changelog entry: clog add [section] [stanza] [message]

• rereviewed Files changed in the github PR explorer

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For Admin Use:

• Added appropriate labels to PR (ex. wip, ready-for-review, docs)
• Reviewers Assigned
• Squashed all commits, uses message "Merge pull request #XYZ: [title]" (coding standards)

[codecov]

Codecov Report

Merging #4755 into master-docs will increase coverage by 0.01%.
The diff coverage is n/a.

@@               Coverage Diff               @@
##           master-docs    #4755      +/-   ##
===============================================
+ Coverage        53.74%   53.75%   +0.01%     
===============================================
  Files              272      272              
  Lines            17103    17103              
===============================================
+ Hits              9192     9194       +2     
+ Misses            7224     7222       -2     
  Partials           687      687

[gamarin2]

Reviewed half, will do the other half tomorrow