Docs for MsgPruneAcknowledgement

[chatton]

Description

pending #5482

closes: #5501

---

Before we can merge this PR, please make sure that all the following items have been
checked off. If any of the checklist items are not applicable, please leave them but
write a little note why.

• Targeted PR against the correct branch (see CONTRIBUTING.md).
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Code follows the module structure standards and Go style guide.
• Wrote unit and integration tests.
• Updated relevant documentation (docs/) or specification (x/<module>/spec/).
• Added relevant godoc comments.
• Provide a commit message to be used for the changelog entry in the PR description for review.
• Re-reviewed Files changed in the Github PR explorer.
• Review Codecov Report in the comment section below once CI passes.

[DimitrisJim]

lgtm! are we fine with CLI being here as opposed to it being in a separate client.md as we do in other places? (Can't find where other CLI commands for core are documented tbh)