Docs: Add note about version counter in ModelCheckpoint

[adosar]

Fixes: #20130

Add a note for the save_top_k parameter, emphasizing that users shouldn't rely on version count to retrieve the best model but instead pass formatting options to filename.

What does this PR do?

Clarifies that users shouldn't rely on version count appended to the filenames of checkpoints for retrieving the best k models.

Fixes #20130

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📚 Documentation preview 📚: https://pytorch-lightning--20146.org.readthedocs.build/en/20146/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81%. Comparing base (194c3c3) to head (e4eaff9).
Report is 3 commits behind head on master.

❗ There is a different number of reports uploaded between BASE (194c3c3) and HEAD (e4eaff9). Click for more details.

HEAD has 90 uploads less than BASE

Flag	BASE (194c3c3)	HEAD (e4eaff9)
gpu	4	2
pytest	25	2
lightning_fabric	7	0
cpu	42	21
python3.9	12	6
lightning	32	16
python3.10	12	6
python3.11	12	6
python3.12	6	3

Additional details and impacted files

@@            Coverage Diff            @@
##           master   #20146     +/-   ##
=========================================
- Coverage      89%      81%     -8%     
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  Files         267      264      -3     
  Lines       23044    22989     -55     
=========================================
- Hits        20546    18582   -1964     
- Misses       2498     4407   +1909