Add docs discussing how to get a matrix of pairwise comparisons

[hyanwong]

this is a common thing to want to do

[codecov]

Codecov Report

Merging #2776 (c400728) into main (3d4fc51) will not change coverage.
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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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[jeromekelleher]

Let's keep this one on ice until we have the pairwise matrix code up to speed. We won't want people to land on this (much slower) way of doing the same thing.

[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.