You should be using the Ask Ubuntu Toolbar Buttons UserScript!
Not only do you get a button in the toolbar for inserting links to manpages, but you also get a button for inserting links to packages and inserting links to PPAs.
For the record, the following markdown is generated by the script for the ping command:
[manpage for the `ping` command ](http://manpages.ubuntu.com/ping)
Rendered as:
