I was messing around with Google this morning, when I realised that you can actually "subscribe" yourself to a custom search for linux manpages. You have to be logged in of course. :-)
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.
Afterwards, you go to Settings ⟶ Search settings.
Next, scroll down to "Subscribed Links" and click on the Subscribe button next to the "Unix Manpages". Click "Save preferences" and you're redirected back to the "man nmap" search. The search result is listed third here, so the result might be different than yours.
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!
- Head on to http://manpages.debian.net.
- 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".
- Name: Manpages, Keyword: man
- Click Save.
- Head to your location bar text box and type in "man nmap".
Voila! Have fun keyword-ing!