Add doxygen document for svc_layercontext

BUG=aomedia:2714

Change-Id: I6a935e98727a3f983debbd28e0073a66dc3e2696
diff --git a/docs.cmake b/docs.cmake
index 875d774..d76dc88 100644
--- a/docs.cmake
+++ b/docs.cmake
@@ -141,6 +141,8 @@
       "${AOM_ROOT}/av1/encoder/rdopt.h"
       "${AOM_ROOT}/av1/encoder/rdopt.c"
       "${AOM_ROOT}/av1/encoder/speed_features.h"
+      "${AOM_ROOT}/av1/encoder/svc_layercontext.c"
+      "${AOM_ROOT}/av1/encoder/svc_layercontext.h"
       "${AOM_ROOT}/av1/encoder/tx_search.h")
 endif()