5.6. Where are the man pages for GNU sed?
Prior to GNU sed v3.02, there weren't any. Until recently, man
pages distributed with gsed were borrowed from old sources or from
other compilations. None of them were "official." GNU sed v3.02 had
the first real set of official man pages, and the documentation has
greatly improved with GNU sed version 4.0, which now includes both
man pages and textinfo pages.