ctest_read_custom_files: mention that ctest does this automatically

Help/command/ctest_read_custom_files.rst

@@ -9,3 +9,6 @@ read CTestCustom files.
 
 Read all the CTestCustom.ctest or CTestCustom.cmake files from the
 given directory.
+
+By default, invoking :manual:`ctest(1)` without a script will read custom
+files from the binary directory.