Camera: clarify unknown error logic in buffer request

Test: N/A. doc update
Bug: 120986771
Change-Id: Ibf4ac8c4618f06f7075ff762f8e1461211cdea7b
diff --git a/include/hardware/camera3.h b/include/hardware/camera3.h
index 03db10c..90d70df 100644
--- a/include/hardware/camera3.h
+++ b/include/hardware/camera3.h
@@ -2200,7 +2200,9 @@
 
     /**
      * request_stream_buffers() call failed for all streams and no buffers are
-     * returned at all due to unknown reason.
+     * returned at all. This can happen for unknown reasons or a combination
+     * of different failure reasons per stream. For the latter case, caller can
+     * check per stream failure reason returned in camera3_stream_buffer_ret.
      */
     CAMERA3_BUF_REQ_FAILED_UNKNOWN = 4,