Clarify the help message for CONFIG_DEBUG

Change the help message for CONFIG_DEBUG from "Debug build flag." to
"Enable debug-only code." The original help message could be
misinterpreted as enabling CMAKE_BUILD_TYPE=Debug.

BUG=aomedia:2866

Change-Id: I20f881c8f4f2ec7b3fbfdf503a5eaab536b1bf55
diff --git a/build/cmake/aom_config_defaults.cmake b/build/cmake/aom_config_defaults.cmake
index 35e5d93..7e21cd5 100644
--- a/build/cmake/aom_config_defaults.cmake
+++ b/build/cmake/aom_config_defaults.cmake
@@ -84,7 +84,7 @@
 set_aom_config_var(CONFIG_WEBM_IO 1 "Enables WebM support.")
 
 # Debugging flags.
-set_aom_config_var(CONFIG_DEBUG 0 "Debug build flag.")
+set_aom_config_var(CONFIG_DEBUG 0 "Enable debug-only code.")
 set_aom_config_var(CONFIG_MISMATCH_DEBUG 0 "Mismatch debugging flag.")
 
 # AV1 feature flags.