Update compatibility.md

Author: gahl-levy

articles/cosmos-db/mongodb/vcore/compatibility.md

@@ -8,7 +8,7 @@ ms.reviewer: gahllevy
 ms.service: cosmos-db
 ms.subservice: mongodb-vcore
 ms.topic: conceptual
-ms.date: 03/09/2023
+ms.date: 04/11/2023
 ---
 
 # MongoDB compatibility and feature support with Azure Cosmos DB for MongoDB vCore
@@ -637,6 +637,9 @@ Azure Cosmos DB for MongoDB supports documents encoded in MongoDB BSON format.
 
 Azure Cosmos DB for MongoDB vCore supports the following indexes and index properties:
 
+> [!NOTE]
+> Creating a **unique index** obtains an exclusive lock on the collection for the entire duration of the build process. This blocks read and write operations on the collection until the operation is completed.
+
 ### Indexes
 
 | Command | Supported |