Updated Azure AD architecture

Update wording in Azure AD docs to reflect each DC having independent read-write consistency.

Author: eketo-msft

articles/active-directory/fundamentals/active-directory-architecture.md

@@ -91,7 +91,7 @@ The directory model is one of eventual consistencies. One typical problem with d
 
 Azure AD provides read-write consistency for applications targeting a secondary replica by routing its writes to the primary replica, and synchronously pulling the writes back to the secondary replica.
 
-Application writes using the Graph API of Azure AD are abstracted from maintaining affinity to a directory replica for read-write consistency. The Azure AD Graph service maintains a logical session, which has affinity to a secondary replica used for reads; affinity is captured in a “replica token” that the graph service caches using a distributed cache. This token is then used for subsequent operations in the same logical session. 
+Application writes using the Graph API of Azure AD are abstracted from maintaining affinity to a directory replica for read-write consistency. The Azure AD Graph service maintains a logical session, which has affinity to a secondary replica used for reads; affinity is captured in a “replica token” that the graph service caches using a distributed cache in the secondary replica datacenter. This token is then used for subsequent operations in the same logical session. To continue using the same logical session, subsequent requests must be routed to the same Azure AD datacenter. It is not possible to continue a logical session if the directory client requests are being routed to multiple Azure AD datacenters; if this happens then the client has multiple logical sessions which have independent read-write consistencies.
 
  >[!NOTE]
  >Writes are immediately replicated to the secondary replica to which the logical session's reads were issued.