fix(flags): document exposure cache capacity

Author: leoromanovsky

features/dd-sdk-android-flags/src/main/kotlin/com/datadog/android/flags/internal/ExposureEventsProcessor.kt

@@ -89,8 +89,10 @@ internal class ExposureEventsProcessor(private val writer: RecordWriter, private
     }
 
     companion object {
-        // Maximum cache size in bytes (8MB)
-        private const val MAX_CACHE_SIZE_BYTES = 8 * 1024 * 1024 // 8MB
+        // Maximum cache size in bytes (4MB). The expected high-water mark is
+        // two subjects with 2,500 flags each, which is about 2.8-3.8MB when
+        // identifiers average 200-300 combined characters per cache entry.
+        private const val MAX_CACHE_SIZE_BYTES = 4 * 1024 * 1024 // 4MB
 
         // Memory overhead constants for size calculation
         private const val OBJECT_OVERHEAD = 16 // bytes for object header