Add note about Windows LONG

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from #20856)

crypto/threads_win.c

@@ -262,6 +262,7 @@ int CRYPTO_atomic_load_int(int *val, int *ret, CRYPTO_RWLOCK *lock)
 
     return 1;
 #else
+    /* On Windows, LONG is always the same size as int. */
     *ret = (int)InterlockedOr((LONG volatile *)val, 0);
     return 1;
 #endif