Document changes in device registration

docs/Deploying-Push-Server.md

@@ -60,6 +60,14 @@ You can enable storing of sent messages in database using following property:
 powerauth.push.service.message.storage.enabled=true
 ```
 
+### Enabling Multiple Associated Activations in Device Registration 
+
+You can enable registration of multiple associated activations for a push token using following property:
+
+```
+powerauth.push.service.registration.multipleActivations.enabled=true
+```
+
 ### APNS Environment Configuration
 
 In order to separate development and production environment on APNS, you may want to set following property:

docs/PowerAuth-Push-Server-0.23.0.md

@@ -23,3 +23,23 @@ CREATE UNIQUE INDEX `push_device_activation` ON `push_device_registration`(`acti
 ```
 
 In case either of the index updates fails, delete existing duplicate rows. Rows with newest timestamp_last_registered should be preserved.
+
+## Device Registration Changes
+
+Following changes of device registration have been applied in release `0.23.0`:
+- A device can no longer be registered with the same `activationId` and multiple related `pushtoken` values. 
+This change was introduced because Google and Apple do not always expire existing push tokens. When the device
+receives a new push token, the device registration endpoint updates the `pushtoken` value for an existing activation instead of
+creating a new device registration. Thus the old push token is removed from database.
+- It is no longer possible to register a device without associated activation. The `activationId` parameter must be
+always sent with device registration request.
+- It is possible to re-register a device with same `activationId` and `pushtoken`. The registration timestamp is updated in this case. 
+- It is possible to register a device with multiple `activationIds` associated with a single `pushtoken`. Such 
+device registration must be initiated using the new endpoint created for this use case: 
+[Create Device for Multiple Associated Activations](./Push-Server-API.md#create-device-for-multiple-associated-activations).
+Multiple activations are used in master-child activation schemes. The functionality needs to be enabled
+using a configuration property [for enabling multiple activations](./Deploying-Push-Server.md#enabling-multiple-associated-activations-in-device-registration),
+because it is less secure than the case when a single associated activation is allowed for a push token.
+- Database indexes are now applied to enforce database consistency for device registrations:
+  - The (`activationId`) value must be unique in the device registration table. Each `activationId` must have exactly one associated `pushtoken`. 
+  - The (`activationId`, `pushtoken`) combination must be unique in the device registration table.