Documentation: describe shallow repository

Signed-off-by: Junio C Hamano <junkio@cox.net>

Documentation/glossary.txt

@@ -286,6 +286,18 @@ SCM::
 SHA1::
 	Synonym for object name.
 
+shallow repository::
+	A shallow repository has an incomplete history some of
+	whose commits have parents cauterized away (in other
+	words, git is told to pretend that these commits do not
+	have the parents, even though they are recorded in the
+	commit object).  This is sometimes useful when you are
+	interested only in the recent history of a project even
+	though the real history recorded in the upstream is
+	much larger.  A shallow repository is created by giving
+	`--depth` option to gitlink:git-clone[1], and its
+	history can be later deepened with gitlink:git-fetch[1].
+
 symref::
 	Symbolic reference: instead of containing the SHA1 id itself, it
 	is of the format 'ref: refs/some/thing' and when referenced, it

Documentation/repository-layout.txt

@@ -18,6 +18,8 @@ could have only commit objects without associated blobs and
 trees this way, for example.  A repository with this kind of
 incomplete object store is not suitable to be published to the
 outside world but sometimes useful for private repository.
+. You also could have an incomplete but locally usable repository
+by cloning shallowly.  See gitlink:git-clone[1].
 . You can be using `objects/info/alternates` mechanism, or
 `$GIT_ALTERNATE_OBJECT_DIRECTORIES` mechanism to 'borrow'
 objects from other object stores.  A repository with this kind
@@ -162,3 +164,9 @@ logs/refs/heads/`name`::
 
 logs/refs/tags/`name`::
 	Records all changes made to the tag named `name`.
+
+shallow::
+	This is similar to `info/grafts` but is internally used
+	and maintained by shallow clone mechanism.  See `--depth`
+	option to gitlink:git-clone[1] and gitlink:git-fetch[1].
+