Со man(1)страницы руководства :
SEARCH PATH FOR MANUAL PAGES man uses a sophisticated method of finding manual page files, based on the invocation options and environment variables, the /etc/man.config configuration file, and some built in conventions and heuristics. First of all, when the name argument to man contains a slash (/), man assumes it is a file specification itself, and there is no searching involved. But in the normal case where name doesn’t contain a slash, man searches a variety of directories for a file that could be a manual page for the topic named.
Там намного больше текста, хотя. Я позволю вам прочитать это на досуге.