Clarify to_excel() documentation regarding overwrite behavior

[tomas-cc]

• closes #xxxx (Replace xxxx with the GitHub issue number)
• Tests added and passed if fixing a bug or adding a new feature
• All code checks passed.
• Added type annotations to new arguments/methods/functions.
• Added an entry in the latest doc/source/whatsnew/vX.X.X.rst file if fixing a bug or adding a new feature.
• If I used AI to develop this pull request, I prompted it to follow AGENTS.md.

[mroeschke]

Is this associated with an open issue?

[tomas-cc]

Hey,
The original wording in the docs was a bit misleading and made me think that Excel files couldn’t be appended. Because of that, I spent time working on unnecessary workarounds.
Later I noticed docs on [ExcelWriter](https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.ExcelWriter.html) says that It can be opened in append mode and has if_sheet_exists handling. And it works after trying that out.
So I though update the docs might prevent someone else from wasting their time.

I can open an issue if its needed and the change looks good.