Every single DebianPackage has its documentation in the directory /usr/doc/PackageName (in releases 2.2 and earlier) or /usr/share/doc/PackageName (in releases later than 2.2).
Each executable command is supposed to have a ManPage. Some packages are documented in ?GNUInfo format. To find out what form a package's documentation is in, try dpkg --listfiles <packagename> (after the package is installed).
If you have a web server installed, try installing dwww . This will allow you to access the documentation for many packages using a web browser.