Camera: Add doc for combined stream usage flags

Specify the HAL behavior when stream usage flag is combination for
multiple consumers.

Test: Compile
Bug: 33777818
Change-Id: I97e7bcc79ee36a395210c7cf8deefc4f175d3e07
diff --git a/camera/device/3.2/ICameraDeviceSession.hal b/camera/device/3.2/ICameraDeviceSession.hal
index e92d756..a4816b4 100644
--- a/camera/device/3.2/ICameraDeviceSession.hal
+++ b/camera/device/3.2/ICameraDeviceSession.hal
@@ -152,6 +152,7 @@
      *             camera3_stream_configuration_t->operation_mode requirements
      *             for non-NORMAL mode, or the requested operation_mode is not
      *             supported by the HAL.
+     *           - Unsupported usage flag
      *         The camera service cannot filter out all possible illegal stream
      *         configurations, since some devices may support more simultaneous
      *         streams or larger stream resolutions than the minimum required