After “make”-ing, the info manual is ready to use. You can run it, for example, with C-u C-h i command (the file is doc/emacs-guix.info). However it is not convenient. You may want to have this manual in the top info directory along with the other manuals. To make it possible, you need to do the following 2 steps:
Alternatively, you can adjust Info path inside Emacs, like this:
(with-eval-after-load 'info (info-initialize) (add-to-list 'Info-directory-list "/path/to/emacs-guix/doc"))
Once the above 2 steps are done, C-h i should display the Emacs-Guix entry. Also you can use M-x guix-info command to run the manual (see Help).