hel of some commands not found

[Michele Bert]

Hello
I am using GNU Emacs 23.3.1 (i686-pc-linux-gnu, X toolkit, Xaw3d
scroll bars) of 2013-05-17 on komainu, modified by Debian, on and
Ubutu 12.04.
and I cannot find help for some commands. For example I have just
looked for command "format-decode-buffer", though I got the "Concept
index".
Where does emacs look for help? I suppose in that case the system is
missing some -doc packedge, but how can I get which one I have to
install?

Thanks
-- 
Mick