Update to include information about using SPDX-License-Identifier

lines.

share/man/man9/style.9

@@ -65,6 +65,8 @@ The copyright header should be a multi-line comment, with the first
 line of the comment having a dash after the star like so:
 .Bd -literal
 /*-
+ * SPDX-License-Identifier: BSD-2-Clause-FreeBSD
+ *
  * Copyright (c) 1984-2025 John Q. Public
  * All rights reserved.
  *
@@ -82,6 +84,7 @@ license or copyright notice, change the dash to a star for those
 comments.
 Comments starting in columns other than the first are never
 considered license statements.
+Use the appropriate SPDX-License-Identifier line before the copyright.
 .Pp
 After any copyright header, there is a blank line, and the
 .Li $\&FreeBSD$