$ sudo apt-get install zsh-doc
[...]
$ man zsh
No manual entry for zsh
See 'man 7 undocumented' for help when manual pages are not available.
$ man 7 undocumented
[...]
NAME
       undocumented - No manpage for this program, utility or function
[...]

Damn you Ubuntu/Debian/whoever decided that man pages were ‘too big’ to be part of documentation packages!