Add documentation from PR description

Author: bmarty

vector/src/main/java/im/vector/app/features/settings/BackgroundSyncMode.kt

@@ -16,9 +16,26 @@
 
 package im.vector.app.features.settings
 
+/**
+ * Different strategies for Background sync, only applicable to F-Droid version of the app
+ */
 enum class BackgroundSyncMode {
+    /**
+     * In this mode background syncs are scheduled via Workers, meaning that the system will have control on the periodicity
+     * of syncs when battery is low or when the phone is idle (sync will occur in allowed maintenance windows). After completion
+     * the sync work will schedule another one.
+     */
     FDROID_BACKGROUND_SYNC_MODE_FOR_BATTERY,
+
+    /**
+     * This mode requires the app to be exempted from battery optimization. Alarms will be launched and will wake up the app
+     * in order to perform the background sync as a foreground service. After completion the service will schedule another alarm
+     */
     FDROID_BACKGROUND_SYNC_MODE_FOR_REALTIME,
+
+    /**
+     * The app won't sync in background
+     */
     FDROID_BACKGROUND_SYNC_MODE_DISABLED;
 
     companion object {