must_exist is incompletely documented and misleading

### Type of issue

Inaccurate

### What documentation page is affected

https://www.elastic.co/docs/manage-data/data-store/aliases

### What happened?

There's two problems, the page states:

> It may be more appropriate to set `must_exist` to `true`, which will cause the entire action list to fail if a single action fails.

1. there is no example of how to use `must_exist`, i.e. where it is valid.
2. "which will cause the entire action list to fail if a single action fails" is **exactly** what I want from multi entry actions, but this seems to be impossible (when I try to do this I get a failure on remove, and a 2nd alias entry incorrectly created. I need a swap where the remove **must** succeed **as specified** for the add to take place)

I suspect point 2 is the docs leading me to believe something is possible which is not actually possible.
point 1 is purely missing information in the docs.

### Additional info

_No response_