Some GNU software comes with "info" pages rather than man pages (or, sometimes they come with both, but the man page is very short and directs the reader to look at the info page for more info).
Info is somewhat controversial, and man vs info is a subject of holy wars. Info is somewhat more difficult to use than man, since man uses the straightforward default pager, while info is based on Emacs. (Emacs is a powerful editor, with a lot of commands, but because of this, is also somewhat difficult for noobs to use at first. Not a good setup for a manual.) On the other hand, info files are easier to write, since they don't make use of the troff markup language. Info was developed by the GNU Project because no libre version of troff (which used to format man pages) existed at the time.