Fix Dataframe.apply documentation to include note regarding calling f…

[alonme]

…unction twice on first element

• closes DOC: Fix Dataframe.apply documentation to include note regarding calling function twice on first element #33879
• tests added / passed
• passes black pandas
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

[WillAyd]

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?

[alonme]

Very interested, however i will need some guidance as i am not familiar with the internals of the project.

[WillAyd]

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