[DOC] Standardize doctest behavior for PPL settings: remove per-doc “rollback” sections

[penghuo]

What do you want to do?

• Request a change to existing documentation
• Add new documentation
• Report a technical problem with the documentation
• Other

Tell us about your request. Provide a summary of the request.

Background

Each PPL settings doc includes a “rollback” section intended to restore defaults so that doctests don’t affect one another. The per-doc rollback also adds noise and duplication.

Proposal

• Remove the rollback section from each settings doc.
• In doctests, apply the setting only in the final step of the example (no intermediate state persists into later checks).

Version: List the OpenSearch version to which this issue applies, e.g. 2.14, 2.12--2.14, or all.
all

What other resources are available? Provide links to related issues, POCs, steps for testing, etc.