add info about device id could be empty

Author: wenxi-zeng

src/connections/sources/catalog/libraries/mobile/android/index.md

@@ -23,7 +23,7 @@ Naturally the Analytics SDK needs a unique ID for each user. The very first time
 
 The Segment SDK also collects the [Advertising ID](https://developer.android.com/google/play-services/id.html) provided by Play Services. Make sure the Play Services Ads library is included as a dependency for your application. This is the ID that should be used for advertising purposes. This value is set to `context.device.advertisingId`.
 
-Segment also generates a unique ID by using the [DRM API](https://source.android.com/devices/drm) as `context.device.id`. Some destinations rely on this field being the Android ID, so be sure to double-check the destination's vendor documentation. If you choose to override the default value, make sure the identifier you choose complies with Google's [User Data Policy](https://support.google.com/googleplay/android-developer/answer/10144311).
+Segment also generates a unique ID by using the [DRM API](https://source.android.com/devices/drm) as `context.device.id`. Some destinations rely on this field being the Android ID, so be sure to double-check the destination's vendor documentation. If you choose to override the default value, make sure the identifier you choose complies with Google's [User Data Policy](https://support.google.com/googleplay/android-developer/answer/10144311). Note that if this id is not previously generated (app is fresh installed or updated from an old version), an empty string is presented before the generation is completed.
 
 > warning ""
 > **Note:** From `4.10.1`, Segment no longer collects the [Android ID](http://developer.android.com/reference/android/provider/Settings.Secure.html#ANDROID_ID) to comply with Google's [User Data Policy](https://support.google.com/googleplay/android-developer/answer/10144311).

src/connections/sources/catalog/libraries/mobile/kotlin-android/index.md

@@ -64,7 +64,7 @@ To get started with the Analytics-Kotlin mobile library:
     `application` | Default set to `null`. <br> The application specific object (in the case of `Android: ApplicationContext`).
     `apiHost` | Default set to `api.segment.io/v1`. <br> This sets a default API Host to which Segment sends events. |
     `autoAddSegmentDestination` | Default set to `true`. <br> This automatically adds the Segment Destination plugin. You can set this to `false` if you want to manually add the Segment Destination plugin. |
-    `collectDeviceId` | Default set to `false`. <br> Set to `true` to automatically collect the device Id. |
+    `collectDeviceId` | Default set to `false`. <br> Set to `true` to automatically collect the device Id. <br> The device id is generated by [DRM API](https://source.android.com/devices/drm). If it is not previously generated (app is fresh installed), an empty string is presented before the generation is completed. You can plug custom id by writing your own [`Plugin`](#plugin) |
     `defaultSettings` | Default set to `{}`. <br> The settings object used as fallback in case of network failure. |
     `flushAt` | Default set to `20`. <br> The count of events at which Segment flushes events. |
     `flushInterval` | Default set to `30` (seconds). <br> The interval in seconds at which Segment flushes events. |