Adding type for bookmark

Author: Oxyjun

src/content/docs/d1/worker-api/d1-database.mdx

@@ -281,4 +281,5 @@ return { bookmark } = session.getBookmark();
 
 #### Return values
 
--
+- <code>bookmark</code>: <Type text="String | null"/>
+  - A [`bookmark`](/d1/reference/time-travel/#bookmarks) which identifies the latest version of the database seen by the write query.