[docs] clarify readable callback timing

#6724

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

@@ -357,7 +357,7 @@ store = derived([a, ...b], callback: ([a: any, ...b: any[]], set: (value: any) =
 
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-Derives a store from one or more other stores. Whenever those dependencies change, the callback runs.
+Derives a store from one or more other stores. Whenever those dependencies change or the first subscriber subscribes, the callback runs.
 
 In the simplest version, `derived` takes a single store, and the callback returns a derived value.