docs: show HOWTO generate in abra man help

See #568

cli/run.go

@@ -73,7 +73,8 @@ func Run(version, commit string) {
 		Aliases: []string{"m"},
 		Short:   "Generate manpage",
 		Example: `  # generate the man pages into /usr/local/share/man/man1
-  sudo abra man
+  abra_path=$(which abra)  # pass abra absolute path to sudo below
+  sudo $abra_path man
   sudo mandb
 
   # read the man pages