CheckC*CompilerFlag: add documentation what to expect from a positive result

Modules/CheckCCompilerFlag.cmake

@@ -6,6 +6,9 @@
 # sets CMAKE_REQUIRED_DEFINITIONS to <flag>.
 # See help for CheckCSourceCompiles for a listing of variables
 # that can otherwise modify the build.
+# The result only tells that the compiler does not give an error message when
+# it encounters the flag. If the flag has any effect or even a specific one is
+# beyond the scope of this module.
 
 #=============================================================================
 # Copyright 2006-2011 Kitware, Inc.

Modules/CheckCXXCompilerFlag.cmake

@@ -6,6 +6,9 @@
 # sets CMAKE_REQUIRED_DEFINITIONS to <flag>.
 # See help for CheckCXXSourceCompiles for a listing of variables
 # that can otherwise modify the build.
+# The result only tells that the compiler does not give an error message when
+# it encounters the flag. If the flag has any effect or even a specific one is
+# beyond the scope of this module.
 
 #=============================================================================
 # Copyright 2006-2010 Kitware, Inc.