Agreed; Emacs is the gold standard for documentation. It comes with a reference manual (400k words), an Emacs Lisp reference (600k words), _and_ 64 other manuals for individual Emacs modes or features including one just for Calc mode (250k words), a manual just for Org mode (130k words), one for Gnus (180k words) etc. All told it adds up to about 2.6 million words of documentation.

Still, another manual written from a different perspective probably won’t hurt anything.