Update docstrings for backward methods

[awaelchli]

What does this PR do?

After recent changes in #13863, I forgot to update the docstrings.

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• Did you write any new necessary tests? (not for typos and docs)
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• Did you update the CHANGELOG? (not for typos, docs, test updates, or internal minor changes/refactorings)

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Did you have fun?

I made sure I had fun coding 🙃

cc @Borda @rohitgr7 @awaelchli @akihironitta

[codecov]

Codecov Report

Merging #13886 (6920ca5) into master (a58a406) will increase coverage by 14%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master   #13886     +/-   ##
=========================================
+ Coverage      61%      76%    +14%     
=========================================
  Files         332      343     +11     
  Lines       26078    27311   +1233     
=========================================
+ Hits        15939    20647   +4708     
+ Misses      10139     6664   -3475     

[kaushikb11]

@tchaton @justusschock

[tchaton]

LGTM !