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 (1bc2aad) to head (2f90a89).
Report is 41 commits behind head on master.

❗ There is a different number of reports uploaded between BASE (1bc2aad) and HEAD (2f90a89). Click for more details.

HEAD has 270 uploads less than BASE

Flag	BASE (1bc2aad)	HEAD (2f90a89)
cpu	82	21
lightning_fabric	13	0
pytest	45	2
python3.9	23	6
lightning	60	16
gpu	4	2
python3.11	24	6
python3.10	24	6
python3.12	11	3
pytorch2.1	24	12
pytorch_lightning	13	7
pytest-full	41	21
pytorch2.2	6	3
pytorch2.3	6	3
pytorch2.4	5	3

Additional details and impacted files

@@            Coverage Diff            @@
##           master   #20146     +/-   ##
=========================================
- Coverage      89%      81%     -8%     
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  Files         267      264      -3     
  Lines       23045    22990     -55     
=========================================
- Hits        20547    18583   -1964     
- Misses       2498     4407   +1909