Add KDoC for convertStreamToString.

Change-Id: Idb7fc085e889f8012234d14ad7bc0a4713073d6e

Author: Jaehwa-Noh

core/network/src/main/kotlin/com/google/samples/apps/nowinandroid/core/network/demo/DemoNiaNetworkDataSource.kt

@@ -77,6 +77,9 @@ class DemoNiaNetworkDataSource @Inject constructor(
     override suspend fun getNewsResourceChangeList(after: Int?): List<NetworkChangeList> =
         getNewsResources().mapToChangeList(NetworkNewsResource::id)
 
+    /**
+     * Convert [InputStream] to [String].
+     */
     private suspend fun convertStreamToString(inputStream: InputStream): String = withContext(
         coroutineContext,
     ) {