docs: clarify context state getter pattern (#18226)

Closes #18022

This updates the context docs to show the getter pattern for state that
may be reassigned, such as primitive state. The new example passes a
function through context so children read the current value instead of
capturing the initial value.

This is a docs-only clarification and does not change runtime behavior.

Validation:

- `git diff --check`
- `pnpm exec prettier --check
documentation/docs/06-runtime/02-context.md`

Full `pnpm test`/`pnpm lint` were not run because this only changes
documentation.

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Co-authored-by: Rich Harris <hello@rich-harris.dev>