Fix how auc scores are calculated in PrecisionRecallCurve.plot methods

[SkafteNicki]

What does this PR do?

Fixes #2405
Because the scores for this metric is not in ascending order but descending the direction argument should be set to -1.0 when calculating the AUC. Additionally, adds a bit of description about how the value is calculated.

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• Did you make sure to update the docs?
• Did you write any new necessary tests?

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📚 Documentation preview 📚: https://torchmetrics--2437.org.readthedocs.build/en/2437/

[codecov]

Codecov Report

Merging #2437 (5e3ed6a) into master (4ed43e6) will not change coverage.
The diff coverage is 75%.

Additional details and impacted files

@@          Coverage Diff           @@
##           master   #2437   +/-   ##
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  Coverage      69%     69%           
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  Files         307     307           
  Lines       17363   17363           
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  Hits        11961   11961           
  Misses       5402    5402