Add missing documentation for Dom.Storage.setItem in v8.0.0

Author: bodhish

pages/docs/manual/v8.0.0/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
 
 ```re sig

pages/docs/manual/v8.0.0/api/dom/storage2.mdx

@@ -18,6 +18,12 @@ type t
 external getItem: (t, string) => option(string) = "getItem"
 ```
 
+## setItem
+
+```res sig
+external setItem: (t, string, string) => unit = "setItem"
+```
+
 ## removeItem
 
 ```re sig