ci: add a comment to run IWYU in verbose mode

Useful to avoid setting up the CI locally to get a single extra message.

.gitlab/ci/configure_debian10_iwyu.cmake

@@ -1,4 +1,6 @@
 set(CMake_RUN_IWYU ON CACHE BOOL "")
+# Uncomment to diagnose IWYU problems as needed.
+#set(CMake_IWYU_VERBOSE ON CACHE BOOL "")
 set(IWYU_COMMAND "/usr/bin/include-what-you-use-6.0" CACHE FILEPATH "")
 
 include("${CMAKE_CURRENT_LIST_DIR}/configure_common.cmake")