Add docstring to HyperparametersMixin

[popfido]

What does this PR do?

Fixes #9102

Does your PR introduce any breaking changes? If yes, please list them.

None

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• Did you list all the breaking changes introduced by this pull request?
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[codecov]

Codecov Report

Merging #9253 (f253514) into master (b046bd0) will decrease coverage by 4%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #9253    +/-   ##
=======================================
- Coverage      92%     88%    -4%     
=======================================
  Files         176     176            
  Lines       14816   14810     -6     
=======================================
- Hits        13669   13051   -618     
- Misses       1147    1759   +612     

[tchaton]

LGMT ! Thanks