原因：
/etc/dpkg/dpkg.cfg.d/excludes文件中，拒绝安装帮助文档。此时，就算apt安装帮助文档显示成功，但实际上并没有安装到本地。excludes文件内容如下：

# Drop all man pages
 path-exclude=/usr/share/man/*

# Drop all documentation ...
 path-exclude=/usr/share/doc/*

# ... except copyright files ...
path-include=/usr/share/doc/*/copyright

# ... and Debian changelogs
path-include=/usr/share/doc/*/changelog.Debian.*

解决：把第2、4行注释掉