Documentation: describe the "repository" in repository-layout

Update the introductory part and concisely explain how gitfile is
handled, what it is used for and for what effect.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/gitrepository-layout.txt

@@ -12,12 +12,24 @@ $GIT_DIR/*
 DESCRIPTION
 -----------
 
-You may find these things in your Git repository (`.git`
-directory for a repository associated with your working tree, or
-`<project>.git` directory for a public 'bare' repository. It is
-also possible to have a working tree where `.git` is a plain
-ASCII file containing `gitdir: <path>`, i.e. the path to the
-real Git repository).
+A Git repository comes in two different flavours:
+
+ * a `.git` directory at the root of the working tree;
+
+ * a `<project>.git` directory that is a 'bare' repository
+   (i.e. without its own working tree), that is typically used for
+   exchanging histories with others by pushing into it and fetching
+   from it.
+
+*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
+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
+submodule working tree, without losing the submodule repository.
+
+These things may exist in a Git repository.
 
 objects::
 	Object store associated with this repository.  Usually