Add documentation about CONFIG_REALTIME_ONLY flag

Bug: aomedia:2865
Change-Id: I218c44f697b463d7e123d9dd3348f4e447a3e04a
diff --git a/aom/aomcx.h b/aom/aomcx.h
index 33f4636..0710a7e 100644
--- a/aom/aomcx.h
+++ b/aom/aomcx.h
@@ -22,6 +22,18 @@
 /*!\file
  * \brief Provides definitions for using AOM or AV1 encoder algorithm within the
  *        aom Codec Interface.
+ *
+ * Several interfaces are excluded with CONFIG_REALTIME_ONLY build:
+ * Global motion
+ * Warped motion
+ * OBMC
+ * TPL model
+ * Loop restoration
+ *
+ * The following features are also disabled with CONFIG_REALTIME_ONLY:
+ * CNN
+ * 4X rectangular blocks
+ * 4X rectangular transform in intra prediction
  */
 
 #ifdef __cplusplus
@@ -376,6 +388,8 @@
    *
    * - 0 = disable
    * - 1 = enable (default)
+   *
+   * \note Excluded from CONFIG_REALTIME_ONLY build.
    */
   AV1E_SET_ENABLE_TPL_MODEL = 35,
 
@@ -639,6 +653,8 @@
    *
    * - 0 = disable
    * - 1 = enable (default)
+   *
+   * \note Excluded from CONFIG_REALTIME_ONLY build.
    */
   AV1E_SET_ENABLE_RESTORATION = 59,
 
@@ -654,6 +670,8 @@
    *
    * - 0 = disable
    * - 1 = enable (default)
+   *
+   * \note Excluded from CONFIG_REALTIME_ONLY build.
    */
   AV1E_SET_ENABLE_OBMC = 61,
 
@@ -983,6 +1001,8 @@
    *
    * - 0 = disable
    * - 1 = enable (default)
+   *
+   * \note Excluded from CONFIG_REALTIME_ONLY build.
    */
   AV1E_SET_ENABLE_GLOBAL_MOTION = 95,
 
@@ -991,6 +1011,8 @@
    *
    * - 0 = disable
    * - 1 = enable (default)
+   *
+   * \note Excluded from CONFIG_REALTIME_ONLY build.
    */
   AV1E_SET_ENABLE_WARPED_MOTION = 96,
 
@@ -1002,6 +1024,8 @@
    *
    * - 0 = disable
    * - 1 = enable (default)
+   *
+   * \note Excluded from CONFIG_REALTIME_ONLY build.
    */
   AV1E_SET_ALLOW_WARPED_MOTION = 97,
 
diff --git a/build/cmake/aom_config_defaults.cmake b/build/cmake/aom_config_defaults.cmake
index 7e21cd5..3bf1eeb 100644
--- a/build/cmake/aom_config_defaults.cmake
+++ b/build/cmake/aom_config_defaults.cmake
@@ -82,6 +82,12 @@
 set_aom_config_var(CONFIG_RUNTIME_CPU_DETECT 1 "Runtime CPU detection support.")
 set_aom_config_var(CONFIG_SHARED 0 "Build shared libs.")
 set_aom_config_var(CONFIG_WEBM_IO 1 "Enables WebM support.")
+set_aom_config_var(CONFIG_REALTIME_ONLY 0
+                   "Build for RTC-only. See aomcx.h for all disabled features.")
+set_aom_config_var(CONFIG_AV1_HIGHBITDEPTH 1
+                   "Build with high bitdepth support.")
+set_aom_config_var(CONFIG_AV1_TEMPORAL_DENOISING 0
+                   "Build with temporal denoising support.")
 
 # Debugging flags.
 set_aom_config_var(CONFIG_DEBUG 0 "Enable debug-only code.")
@@ -127,12 +133,6 @@
                    "Collect encoding component timing information.")
 set_aom_config_var(CONFIG_LPF_MASK 0
                    "Enable the use loop filter bitmasks for optimizations.")
-set_aom_config_var(CONFIG_REALTIME_ONLY 0
-                   "Build for RTC-only to reduce binary size.")
-set_aom_config_var(CONFIG_AV1_HIGHBITDEPTH 1
-                   "Build with high bitdepth support.")
-set_aom_config_var(CONFIG_AV1_TEMPORAL_DENOISING 0
-                   "Build with temporal denoising support.")
 set_aom_config_var(CONFIG_NN_V2 0 "Fully-connected neural nets ver.2.")
 set_aom_config_var(CONFIG_OPTICAL_FLOW_API 0
                    "AV1 experiment flag for optical flow API.")