doc: Document IWYU workaround

src/core_io.cpp

@@ -10,6 +10,9 @@
 #include <consensus/consensus.h>
 #include <consensus/validation.h>
 #include <key_io.h>
+// IWYU incorrectly suggests replacing this header
+// with forward declarations.
+// See https://github.com/include-what-you-use/include-what-you-use/issues/1886.
 #include <primitives/block.h> // IWYU pragma: keep
 #include <primitives/transaction.h>
 #include <script/descriptor.h>