I am running natty and the man page is included in the package
The content of the page is
apt - Advanced Package Tool
APT is a management system for software packages. For normal
day to day
package management there are several frontends available, such
tude(8) for the command line or synaptic(8) for the X Window
Some options are only implemented in apt-get(8) though.
apt-cache(8), apt-get(8), apt.conf(5), sources.list(5),
apt returns zero on normal operation, decimal 100 on error.
This manpage isn't even started.
See <http://bugs.debian.org/apt>. If you wish to report a bug
please see /usr/share/doc/debian/bug-reporting.txt or the
apt was written by the APT team .
Debian GNU/Linux 16 June 1998
So basically, if you have the man pages for apt-cache, apt-get, apt.conf, sources.list, apt-prefer-ences and apt-secure, you have the documentation.
See also : online man page