doc: remove For ... comments

We don't add or maintain these, and they are of little value, as
well as having the effect of polluting diffs.

They are also wrong, i.e DEFAULT_SCRIPTCHECK_THREADS is not in
validation.h.

src/test/util_tests.cpp

@@ -3,9 +3,9 @@
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
 #include <clientversion.h>
-#include <common/signmessage.h> // For MessageSign(), MessageVerify(), MESSAGE_MAGIC
-#include <hash.h> // For Hash()
-#include <key.h>  // For CKey
+#include <common/signmessage.h>
+#include <hash.h>
+#include <key.h>
 #include <script/parsing.h>
 #include <span.h>
 #include <sync.h>
@@ -26,12 +26,13 @@
 
 #include <array>
 #include <cmath>
+#include <cstdint>
+#include <cstring>
 #include <fstream>
 #include <limits>
 #include <map>
 #include <optional>
-#include <stdint.h>
-#include <string.h>
+#include <string>
 #include <thread>
 #include <univalue.h>
 #include <utility>