reactUtils [nfc]: Add caveat to useHas…ForMs about value changing

Based on this discussion thread:
  #5489 (comment)

Author: gnprice

src/reactUtils.js

@@ -62,6 +62,11 @@ export function useDebugAssertConstant<T>(value: T) {
  *
  * The caller must use a constant `duration` through the lifetime of a given
  * component instance.
+ *
+ * Note this hook doesn't (and can't) do anything to cause a rerender when
+ * `value` changes.  The caller must ensure that the component rerenders (so
+ * that in particular this hook gets called again) whenever `value` will
+ * have changed; for example, by using a prop or a `useState` value.
  */
 export function useHasNotChangedForMs(value: mixed, duration: number): boolean {
   useDebugAssertConstant(duration);
@@ -97,6 +102,11 @@ export function useHasNotChangedForMs(value: mixed, duration: number): boolean {
  *
  * The caller must use a constant `duration` through the lifetime of a given
  * component instance.
+ *
+ * Note this hook doesn't (and can't) do anything to cause a rerender when
+ * `value` changes.  The caller must ensure that the component rerenders (so
+ * that in particular this hook gets called again) whenever `value` will
+ * have changed; for example, by using a prop or a `useState` value.
  */
 export const useHasStayedTrueForMs = (value: boolean, duration: number): boolean => {
   useDebugAssertConstant(duration);