Explanation
According to the FHS (Filesystem Hierarchy Standard), the /usr/share/doc directory is used to store documentation files for various packages installed on the system. The documentation files may include README files, manuals, license agreements, changelogs, etc. The /usr/share/doc directory is intended to be read-only and independent of the machine architecture. The other directories are not compliant with the FHS or do not exist by default. The /usr/share/documentation directory is not a standard directory and may not be recognized by some applications. The /usr/local/share/documentation directory is also not a standard directory and may conflict with the /usr/local/share/doc directory, which is used for documentation files for locally installed packages. The /var/share/doc directory does not exist by default and is not a valid subdirectory of
/var, which is used for variable data files. The /etc/share/doc directory does not exist by default and is not a valid subdirectory of /etc, which is used for configuration files. References:
* LPI 101-500 Exam Objectives, Topic 101.3, Weight 2
* LPI Learning Materials, Chapter 1.3, Finding Linux Documentation
* Web Search Results, 1