vsock: document write-once behavior of the child_ns_mode sysctl

Update the vsock child_ns_mode documentation to include the new
write-once semantics of setting child_ns_mode. The semantics are
implemented in a preceding patch in this series.

Signed-off-by: Bobby Eshleman <bobbyeshleman@meta.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Link: https://patch.msgid.link/20260223-vsock-ns-write-once-v3-3-c0cde6959923@meta.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Author: beshleman

Documentation/admin-guide/sysctl/net.rst

@@ -594,6 +594,9 @@ Values:
 	  their sockets will only be able to connect within their own
 	  namespace.
 
+The first write to ``child_ns_mode`` locks its value. Subsequent writes of the
+same value succeed, but writing a different value returns ``-EBUSY``.
+
 Changing ``child_ns_mode`` only affects namespaces created after the change;
 it does not modify the current namespace or any existing children.