DOC: DataFrame.update doctests


I felt this original documentation was doing too much in this code block - the description doesn't match the second code example.

<kbd><img width="762" alt="image" src="https://github.com/pandas-dev/pandas/assets/122238526/cd897229-5366-40d9-8034-5521b7ecd0e7"></kbd>


So I broke it out into these two separate examples:

<kbd><img width="974" alt="image" src="https://github.com/pandas-dev/pandas/assets/122238526/36585ddc-d14b-485e-917e-e53f5609c4c3"></kbd>

---

I added this new example, which I would have found useful:

<kbd><img width="935" alt="image" src="https://github.com/pandas-dev/pandas/assets/122238526/b82fffd1-c095-4784-a2ec-b764b9093a29"></kbd>



- [ ] closes #xxxx (Replace xxxx with the GitHub issue number)
- [ ] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [ ] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) 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.

A lot clearer here. Thanks @smij720 