Also mention -- flag and ignore if undocumented

Reviewed-by: Paul Dale <paul.dale@oracle.com>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/10077)

apps/list.c

@@ -337,6 +337,8 @@ static void list_options_for_command(const char *command)
             continue;
         BIO_printf(bio_out, "%s %c\n", o->name, o->valtype);
     }
+    /* Always output the -- marker since it is sometimes documented. */
+    BIO_printf(bio_out, "- -\n");
 }
 
 static void list_type(FUNC_TYPE ft, int one)

util/find-doc-nits

@@ -809,6 +809,7 @@ sub checkflags {
     }
     if ( scalar @undocced > 0 ) {
         foreach ( @undocced ) {
+            next if /-/; # Skip the -- end-of-flags marker
             err("$doc: undocumented option -$_");
         }
     }