docs: enhance the description of onSubmit()

Author: Mini-ghost

docs/.vitepress/theme/styles/main.css

@@ -14,3 +14,7 @@ iframe {
 .VPFeature:hover {
   border-color: var(--vp-button-brand-border);
 }
+
+.custom-block-title {
+  text-transform: uppercase;
+}

docs/api/use-form.md

@@ -226,8 +226,8 @@ This is your form submission handler, witch will pass your form's `values`. But
   function onSubmit (values: Values, helper: FormSubmitHelper): void | Promise<any>
   ```
 
-::: warning Note
-If `onSubmit()` function is synchronous, you need to call `setSubmitting(false)` yourself.
+::: warning Important
+When the `onSubmit()` function is asynchronous, the `isSubmitting` variable is automatically reset to `false` upon completion. Conversely, if `onSubmit()` is synchronous, you must manually call `setSubmitting(false)` to reset `isSubmitting`.
 :::
 
 ### onInvalid