Fix axes in confusion matrix

[SkafteNicki]

What does this PR do?

Fixes #1971
Switch the order on the axis when plotting confusion matrix such that it is correct.
Also added some text to describe the structure of the confmats (because I keep forgetting myself).

Before submitting

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• Did you read the contributor guideline, Pull Request section?
• Did you make sure to update the docs?
• Did you write any new necessary tests?

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📚 Documentation preview 📚: https://torchmetrics--1976.org.readthedocs.build/en/1976/

[codecov]

Codecov Report

Merging #1976 (89ab95e) into master (cd7ef55) will not change coverage.
The diff coverage is 100%.

Additional details and impacted files

@@          Coverage Diff           @@
##           master   #1976   +/-   ##
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  Coverage      86%     86%           
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  Files         266     266           
  Lines       15185   15185           
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  Hits        13131   13131           
  Misses       2054    2054