doc: update windows -fstack-clash-protection doc

Now that changes have been made in GCC, to fix the build failures.
See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90458.

Author: fanquake

configure.ac

@@ -958,7 +958,8 @@ if test "$use_hardening" != "no"; then
 
   case $host in
     *mingw*)
-      dnl stack-clash-protection doesn't currently work, and likely should just be skipped for Windows.
+      dnl stack-clash-protection doesn't compile with GCC 10 and earlier.
+      dnl In any case, it is a no-op for Windows.
       dnl See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90458 for more details.
       ;;
     *)