From: Philippe Blain <levraiphilippeblain@gmail.com>

'gitmodules.txt' is a guide about the '.gitmodules' file that describes
submodules properties, and that file must exist at the root of the
repository. This was clarified in e5b5c1d2cf (Document clarification:
gitmodules, gitattributes, 2008-08-31).

However, that commit mistakenly uses the non-existing environment
variable 'GIT_WORK_DIR' to refer to the root of the repository.

Fix that by using the correct variable, 'GIT_WORK_TREE'. Take the
opportunity to modernize and improve the formatting of that guide.

Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>

Submitted-As: https://lore.kernel.org/git/pull.942.git.git.1609616245412.gitgitgadget@gmail.com