Add docs discussing how to get a matrix of pairwise comparisons

[hyanwong]

this is a common thing to want to do

[codecov[bot]]

Codecov Report

Merging #2776 (c400728) into main (3d4fc51) will not change coverage. The diff coverage is n/a.

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##             main    #2776   +/-   ##
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  Coverage   89.83%   89.83%           
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  Files          30       30           
  Lines       28624    28624           
  Branches     5590     5590           
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  Hits        25713    25713           
  Misses       1655     1655           
  Partials     1256     1256           

Flag	Coverage Δ
c-tests	86.24% <ø> (ø)
lwt-tests	80.13% <ø> (ø)
python-c-tests	67.13% <ø> (ø)
python-tests	99.01% <ø> (ø)

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[hyanwong]

Fair point. I guess the API would be different then.

[petrelharp]

Wait darn it I was moving too fast - I thought this was an issue, so edited it to reflect a bigger thing that I wanted to do. Apologies, I"ll undo that.