change explanation message in `ABSL_USE_SYSTEM_INCLUDES` option

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 6f96c28..2753c63 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt

@@ -71,7 +71,7 @@
 endif()
 
 option(ABSL_USE_SYSTEM_INCLUDES
-  "Mark C++ include directories with `SYSTEM` to prevent warnings from headers when using Abseil with incompatible warning flags"
+  "Silence warnings in Abseil headers by marking them as `SYSTEM` includes"
   OFF)
 
 list(APPEND CMAKE_MODULE_PATH