man/io_uring_enter.2: add another ENOTSUPP instance
@@ -421,3 +421,7 @@ the io_uring instance is in the process of being torn down
.BR EOPNOTSUPP
.I fd
does not refer to an io_uring instance
+.TP
+.BR EOPNOTSUPP
+.I opcode
+is valid, but not supported by this kernel.
