add docs for for SharedCycleIteratorState

[awaelchli]

What does this PR do?

Addresses comments missed in #8889 by reviewer.

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

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Did you have fun?

I made sure I had fun coding 🙃

[codecov]

Codecov Report

Merging #8952 (0736ef5) into master (413f7b2) will decrease coverage by 4%.
The diff coverage is 0%.

@@           Coverage Diff           @@
##           master   #8952    +/-   ##
=======================================
- Coverage      93%     89%    -4%     
=======================================
  Files         176     176            
  Lines       14411   14411            
=======================================
- Hits        13352   12772   -580     
- Misses       1059    1639   +580     

[tchaton]

LGTM !