cmake: better explain why headertests exists

tests/CMakeLists.txt

@@ -80,7 +80,12 @@ add_clar_test(proxy               -v -sonline::clone::proxy)
 add_clar_test(auth_clone          -v -sonline::clone::cred)
 add_clar_test(auth_clone_and_push -v -sonline::clone::push -sonline::push)
 
-# Header file validation project
+#
+# Header file validation project: ensure that we do not publish any sloppy
+# definitions in our headers and that a consumer can include <git2.dll>
+# even when they have aggressive C90 warnings enabled.
+#
+
 add_executable(headertest headertest.c)
 set_target_properties(headertest PROPERTIES C_STANDARD 90)
 set_target_properties(headertest PROPERTIES C_EXTENSIONS OFF)