Improve docs of isNonNullObject

packages/utils/src/typechecks.ts

@@ -1,5 +1,10 @@
 /**
- * Checks if data is a non-null object (i.e. matches the TypeScript object type)
+ * Checks if data is a non-null object (i.e. matches the TypeScript object type).
+ *
+ * Note: this returns true for arrays, which are objects in JavaScript
+ * even though array and object are different types in JSON.
+ *
+ * @see https://www.typescriptlang.org/docs/handbook/basic-types.html#object
  */
 export function isNonNullObject(data: unknown): data is object {
   return typeof data === "object" && data !== null;