Clarify usage of scrollable segments

Editor assessment

The subject states the documentation goal precisely: usage of scrollable segments is clarified, which names the specific feature whose documentation confused readers. docs(segment) is correct and the scope exact. No body, but the 'resolves #20692' footer is well-formed and links the confusion report that identified the unclear passage, so the motivation is chained. Wording is compact and imperative. A small docs message with its evidence attached properly.