arp.8: don't document a syntax that doesn't exist

[llfw]

The arp.8 manpage documents 'arp -d pub', but the 'pub' flag is not accepted by the arp command. Remove this incorrect documentation.

[bsdimp]

"pub" is still in the sources:

                } else if (strcmp(argv[0], "pub") == 0) {
			opts.flags |= RTF_ANNOUNCE;
                        if (argc && strcmp(argv[1], "only") == 0) {
                                /*                                                                                                                                                                                                          
                                 * Compatibility: in pre FreeBSD 8 times                                                                                                                                                                    
                                 * the "only" keyword used to mean that                                                                                                                                                                     
                                 * an ARP entry should be announced, but                                                                                                                                                                    
                                 * not installed into routing table.                                                                                                                                                                        
                                 */
                                argc--; argv++;
                        }
                } else if (strcmp(argv[0], "blackhole") == 0) {

so why isn't it accepted?

3f844a2 added it, but I haven't seen anything that removed it, at least on purpose.

Ah, looks like 9711a16 removed it with the code that parsed the command line option. I wonder what @glebius was thinking. Tagged him for comment to see if this is a doc bug or a code bug... But it's been like this since 2013 :(.

[llfw]

right, it's still accepted for arp -s, but not for arp -d.

[glebius]

This is a doc bug and submission is correct. Thanks!

P.S. I'm having a hard time looking for "Approve" button in GitHub Web UI.

[markpeek]

@glebius Files Changed tab > Review Changes button > Approve

[llfw]

pushed a minor change to replace "This -d flag" with "The -d flag" which is more standard English.

[bsdimp]

OK. The asymmetry is not a bug, so the doc change is good.