docs: add warning on usage of ssrRef in factory functions

Author: danielroe

docs/helpers/ssrRef.md

@@ -24,3 +24,25 @@ const val2 = ssrRef(myExpensiveSetterFunction)
 ::: tip
 Under the hood, `ssrRef` requires a key to ensure that the ref values match between client and server. If you have added `nuxt-composition-api` to your `buildModules`, this will be done automagically by an injected Babel plugin. If you need to do things differently, you can specify a key manually or add `nuxt-composition-api/babel` to your Babel plugins.
 :::
+
+::: warning
+At the moment, an `ssrRef` is only suitable for one-offs, unless you provide your own unique key.
+
+This is because server and client `ssrRef` matches up based on line number within your code.
+
+```ts
+function useMyFeature() {
+  // Only one unique key is generated
+  const feature = ssrRef('')
+  return feature
+}
+
+const a = useMyFeature()
+const b = useMyFeature()
+
+b.value = 'changed'
+// On client-side, a's value will also be initialised to 'changed'
+```
+
+If you want to use this pattern, make sure to set a unique key based on each calling of the function.
+:::