Improve `useState` with shared composable

Editor assessment

The subject states the improvement and its technique — `useState` documentation now shows the shared composable pattern — so the header communicates what the docs addition teaches without the diff, and the docs type fits. Hygiene is clean: imperative mood, no trailing period, sensible length, no ticket junk. No body is needed for a focused example improvement whose subject carries the approach, and no footer is required. A line on the problem the shared pattern solves, such as cross-component state, would add polish; the message is clear and publishable.