describe configuration variables in /etc/default/grub in comments

[Karl-Philipp Richter]

Some configuration variables like `GRUB_DEFAULT` have a complex structure (It can take a number or a string. What does the number mean if submenues are involved? What names can be specified (`menuitem` labels? How about non-disjoint submenu labels? `menu_option_id`s?). I'm sure that it's documented somewhere, but that's unhelpful if it isn't at least linked (better shortly summaried) at the place where it's needed.

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: grub-efi 2.02~beta2-36
Uname: Linux 4.2.5-040205-generic x86_64
ApportVersion: 2.20-0ubuntu3
Architecture: amd64
Date: Fri Mar 4 14:13:36 2016
InstallationDate: Installed on 2015-04-20 (318 days ago)
InstallationMedia: Ubuntu-Server 14.10 "Utopic Unicorn" - Release amd64 (20141022.2)
ProcEnviron:
 TERM=screen.linux
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=de_DE.UTF-8
 SHELL=/bin/bash
SourcePackage: grub2
UpgradeStatus: Upgraded to xenial on 2016-03-03 (0 days ago)