The grub-reboot(8) man page says:

       The full documentation for grub-reboot is maintained as a
       Texinfo manual.  If the info and grub-reboot programs are
       properly installed at your site, the command

              info grub-reboot

       should give you access to the complete manual.

but "info grub-reboot" just gives the man page.

In bug 1024364:

This is exactly the same bug as 974219. Merging.
And still not fixed yet.

Same issue with other grub2 man pages: grub-editenv, grub-file, etc.

Not all of them: the grub-install(8) man page says

  The  full  documentation  for  grub-install  is  maintained as a Texinfo
  manual.  If the info and grub-install programs are properly installed at
  your site, the command

              info grub-install

  should give you access to the complete manual.

and "info grub-install" opens the grub manual at the node
"24.1 Invoking grub-install", as expected. So, it is possible
that the man pages are correct, but that it is the info file
that has missing entries.

About that, /usr/share/info/grub.info.gz just contains

START-INFO-DIR-ENTRY
* GRUB: (grub).                 The GRand Unified Bootloader
* grub-install: (grub)Invoking grub-install.    Install GRUB on your drive
* grub-mkconfig: (grub)Invoking grub-mkconfig.  Generate GRUB configuration
* grub-mkpasswd-pbkdf2: (grub)Invoking grub-mkpasswd-pbkdf2.
* grub-mkrelpath: (grub)Invoking grub-mkrelpath.
* grub-mkrescue: (grub)Invoking grub-mkrescue.  Make a GRUB rescue image
* grub-mount: (grub)Invoking grub-mount.        Mount a file system using GRUB
* grub-probe: (grub)Invoking grub-probe.        Probe device information
* grub-script-check: (grub)Invoking grub-script-check.
END-INFO-DIR-ENTRY