Add missing documentation for Dom.Storage.setItem

Author: bodhish

pages/docs/manual/latest/api/dom/storage.mdx

@@ -12,6 +12,12 @@ type t
 external getItem: (string, t) => option<string> = "getItem"
 ```
 
+## setItem
+
+```res sig
+external setItem: (string, string, t) => unit = "setItem"
+```
+
 ## removeItem
 
 ```res sig

pages/docs/manual/latest/api/dom/storage2.mdx

@@ -2,10 +2,12 @@
 
 <Intro>
 
+
 The same as [Dom.Storage](./storage2), but with `t` on first argument position for proper `->` usage.
 
 </Intro>
 
+
 ## t
 
 ```res prelude sig
@@ -18,6 +20,12 @@ type t
 external getItem: (t, string) => option<string> = "getItem"
 ```
 
+## setItem
+
+```res sig
+external setItem: (t, string, string) => unit = "setItem"
+```
+
 ## removeItem
 
 ```res sig