Fix Dataframe.apply documentation to include note regarding calling f…
…unction twice on first element

- [ ] closes #33879
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

So looks like there is confusion over the fact that DataFrameGroupBy.apply only calls the function once but DataFrame.apply still calls twice right? Any interest in just trying to fix the latter? Very interested, however i will need some guidance as i am not familiar with the internals of the project. The fix to this on the GroupBy side was #24748 which may be helpful for investigation. Otherwise take a look at the apply implementation in pandas.core.frame 