According to the Filesystem Hierarchy Standard (FHS), the directory /usr/share/man contains manual pages for user commands, system calls, library functions, and other documentation1. The other directories are either non-standard, deprecated, or used for different purposes. For example, /opt/man is used for manual pages for add-on application software packages1, /usr/doc/ is an old location for documentation files that is no longer used2, /var/pkg/man and /var/man are not defined by the FHS. References:
* [LPI Linux Essentials - 1.6 Basic File Editing]
* [LPI Linux Essentials - 1.7 Personalize and/or Localize Your Linux System]
* [LPI Linux Essentials - 1.8 Basic Security and File Permissions]