docs: clarify that observable() mutates input by reference

src/observable.ts

@@ -3,6 +3,36 @@ import { ObservablePrimitiveClass } from './ObservablePrimitive';
 import { createObservable } from './createObservable';
 import type { Observable, ObservablePrimitive, RecursiveValueOrFunction } from './observableTypes';
 
+/**
+ * Create an observable from an initial value.
+ *
+ * **Important:** when `value` is a plain object or array, it is stored **by reference** and
+ * mutated in place as fields are updated via `.set()`. After the first child set, the
+ * variable you passed in no longer holds the original state — it holds the current state.
+ *
+ * This is intentional (it's how Legend State avoids cloning on every update), but it
+ * surprises people coming from Zustand/Redux/MobX. The most common gotcha is using a
+ * shared `initialState` constant as a "reset target":
+ *
+ * ```ts
+ * const initialState = { count: 0 };
+ * const store$ = observable(initialState);
+ *
+ * store$.count.set(5);
+ * console.log(initialState); // { count: 5 } ← mutated in place
+ *
+ * store$.set(initialState);  // ❌ no-op: structurally equal to current value
+ * ```
+ *
+ * If you want a stable "reset target", pass a fresh object/literal each time — typically
+ * via a factory:
+ *
+ * ```ts
+ * const createInitialState = () => ({ count: 0 });
+ * const store$ = observable(createInitialState());
+ * store$.set(createInitialState()); // ✅ fresh object, set fires
+ * ```
+ */
 export function observable<T>(): Observable<T | undefined>;
 export function observable<T>(
     value: Promise<RecursiveValueOrFunction<T>> | (() => RecursiveValueOrFunction<T>) | RecursiveValueOrFunction<T>,