docs: describe exceptional behavior of onSuccess of useMutation.mutate (TanStack#1614)

Author: JungHoonPark

docs/src/pages/reference/useMutation.md

@@ -77,6 +77,7 @@ mutate(variables, {
     - Optional
     - The variables object to pass to the `mutationFn`.
   - Remaining options extend the same options described above in the `useMutation` hook.
+  - If you make multiple requests, `onSuccess` will fire only after the latest call you've made.
 - `mutateAsync: (variables: TVariables, { onSuccess, onSettled, onError }) => Promise<TData>`
   - Similar to `mutate` but returns a promise which can be awaited.
 - `status: string`