Not necessary, but I also think increased doxygen adherence would be better since it's generally used in this header's documentation anyway.
Also, I think highlighting that the function can be called multiple times (through the serialization framework) would be helpful. And we don't do this elsewhere, but the current documentation doesn't say that failure must be non-zero, so could make that explicit (even if most people would assume that already).
My best effort docstring:
<details>
<summary>git diff on ec4ec91d59</summary>
diff --git a/src/kernel/bitcoinkernel.h b/src/kernel/bitcoinkernel.h
index 57c88c1279..761959e9d1 100644
--- a/src/kernel/bitcoinkernel.h
+++ b/src/kernel/bitcoinkernel.h
@@ -347,10 +347,17 @@ typedef void (*btck_ValidationInterfacePoWValidBlock)(void* user_data, btck_Bloc
typedef void (*btck_ValidationInterfaceBlockConnected)(void* user_data, btck_Block* block, const btck_BlockTreeEntry* entry);
typedef void (*btck_ValidationInterfaceBlockDisconnected)(void* user_data, btck_Block* block, const btck_BlockTreeEntry* entry);
-/**
- * Function signature for serializing data.
- *
- * Returns 0 to indicate success.
+/**
+ * [@brief](/github-metadata-backup-bitcoin-bitcoin/contributor/brief/) Callback for writing serialized byte data.
+ *
+ * May be called multiple times for a single serialization. Implementations
+ * should append rather than overwrite previously written data.
+ *
+ * [@param](/github-metadata-backup-bitcoin-bitcoin/contributor/param/)[in] bytes Non-null, pointer to the data to write.
+ * [@param](/github-metadata-backup-bitcoin-bitcoin/contributor/param/)[in] size Number of bytes to write.
+ * [@param](/github-metadata-backup-bitcoin-bitcoin/contributor/param/)[in] userdata Nullable, holds a user-defined opaque structure, passed
+ * through from the calling serialization function.
+ * [@return](/github-metadata-backup-bitcoin-bitcoin/contributor/return/) 0 on success, non-zero on failure.
*/
typedef int (*btck_WriteBytes)(const void* bytes, size_t size, void* userdata);
</details>
