Document preserve_existing parameter

[gmoskovicz]

In #17223 we added a new parameter to preserve existing settings when doing setting update. If set to true existing settings on an index remain unchanged, the default is false.

It would be great to document this setting 😃 .

[s1monw]

++

[glefloch]

Hi, I created a PR that document the new setting. Can you tell me if there is other places where it should be documented?

[javanna]

Closed by #23971