iwyu, doc: Document IWYU pragma: export for <chrono>

src/kernel/mempool_entry.h

@@ -13,8 +13,8 @@
 #include <primitives/transaction.h>
 #include <txgraph.h>
 #include <util/overflow.h>
+#include <util/time.h>
 
-#include <chrono>
 #include <cstddef>
 #include <cstdint>
 #include <functional>

src/kernel/mempool_options.h

@@ -8,8 +8,8 @@
 
 #include <policy/feerate.h>
 #include <policy/policy.h>
+#include <util/time.h>
 
-#include <chrono>
 #include <cstdint>
 #include <optional>
 

src/util/time.h

@@ -6,6 +6,7 @@
 #ifndef BITCOIN_UTIL_TIME_H
 #define BITCOIN_UTIL_TIME_H
 
+// The `util/time.h` header is designed to be a drop-in replacement for `chrono`.
 #include <chrono> // IWYU pragma: export
 #include <cstdint>
 #include <optional>