I say "subscribed" in quotes, because it's not really a subscription, but rather a pointless (my opinion) feature. The only good thing about it is the icon that you notice directly what you want to search, giving a simple explanation of what the command does - in other words, the title of the manpage.
The steps are quite easy! You go to http://www.google.com/search?q=man+nmap and press the "Sign in" link on the top right corner. You log in with your Google account, then you are redirected back to the Google search page.


Finally, let me justify why I consider this pointless. If you use Firefox, you can easily integrate a quick search keyword with your favourite operating system's manpages website. Not all of them have this feature/website, but Debian and Ubuntu have it!

- Select Man -> All sections -> Debian Sid.
- Right-click in the white text box next to "Man Page or Keyword Search".
- Select "Add a keyword for this search".

- Click Save.
- Head to your location bar text box and type in "man nmap".
Voila! Have fun keyword-ing!