doc: make the gitfile syntax easier to discover

Signed-off-by: Marcel Krause <mk+copyleft@pimpmybyte.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/gitrepository-layout.txt

@@ -23,7 +23,9 @@ A Git repository comes in two different flavours:
 
 *Note*: Also you can have a plain text file `.git` at the root of
 your working tree, containing `gitdir: <path>` to point at the real
-directory that has the repository.  This mechanism is often used for
+directory that has the repository.
+This mechanism is called a 'gitfile' and is usually managed via the
+`git submodule` and `git worktree` commands. It is often used for
 a working tree of a submodule checkout, to allow you in the
 containing superproject to `git checkout` a branch that does not
 have the submodule.  The `checkout` has to remove the entire

Documentation/glossary-content.txt

@@ -184,6 +184,8 @@ current branch integrates with) obviously do not work, as there is no
 [[def_gitfile]]gitfile::
 	A plain file `.git` at the root of a working tree that
 	points at the directory that is the real repository.
+	For proper use see linkgit:git-worktree[1] or linkgit:git-submodule[1].
+	For syntax see linkgit:gitrepository-layout[5].
 
 [[def_grafts]]grafts::
 	Grafts enables two otherwise different lines of development to be joined