docs: Add missing docs for CommandInterceptResult fields (#39676)

Document the `string` and `positions` fields to resolve TODO comments.

Release Notes:

- N/A

Signed-off-by: Xiaobo Liu <cppcoffee@gmail.com>
c7d5afedc5Xiaobo Liu committed on 10/8/2025, 5:20:50 PM· committed by GitHubparentd6b1801
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