The help documentation in Unix/Linux is organized like an old-school paper manual. The files of the manual are located in /usr/share/man.
The manual is divided into sections. The number of sections, which is usually 9, and their description can be found using man man.
Each section holds the man pages that are associated with that section. For example, the man pages of section 2 are located inside /usr/share/man/man2.
Each section has an introduction that gives a longer description of that section. This information is in the intro man page of that section. For example, to learn more about section 2, try the command: man 2 intro
To list all the man pages in a section, for example in section 2, use this command: apropos -s 2 .
To generally search for man pages having a certain string, use the apropos command.
There might be many man pages with the same name in different sections. To specify which one you want to view: man 2 open
Linux man pages are updated every few months with new information. The latest Linux man pages can be accessed online at man7.org. The release notes and changelog can also be found online.
OpenBSD is known for the quality of its man pages and they are much more readable than Linux man pages. They can be accessed online here.