kernel: doc: explain return value for btck_WriteBytes callback

This is the only non-void callback type, so it makes sense to document what
implementers should return.

src/kernel/bitcoinkernel.h

@@ -349,6 +349,8 @@ typedef void (*btck_ValidationInterfaceBlockDisconnected)(void* user_data, btck_
 
 /**
  * Function signature for serializing data.
+ *
+ * Returns 0 to indicate success.
  */
 typedef int (*btck_WriteBytes)(const void* bytes, size_t size, void* userdata);