document new spring options

site/content/docs/03-run-time.md

@@ -486,6 +486,8 @@ A `spring` store gradually changes to its target value based on its `stiffness`
 
 As with [`tweened`](#tweened) stores, `set` and `update` return a Promise that resolves if the spring settles. The `store.stiffness` and `store.damping` properties can be changed while the spring is in motion, and will take immediate effect.
 
+Both `set` and `update` can take a second argument — an object with `hard` or `soft` properties. `{ hard: true }` sets the target value immediately; `{ soft: n }` preserves existing momentum for `n` seconds before settling. `{ soft: true }` is equivalent to `{ soft: 0.5 }`.
+
 [See a full example on the spring tutorial.](tutorial/spring)
 
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