doc: glossary: link four of the terms to gitdatamodel(7)

Four of the terms in the glossary are discussed in gitdatamodel(7).
Let’s link to the data model page from the glossary.

The phrasing needs to be tweaked based on what gitdatamodel(7) offers
for each term compared to the glossary, or even other pages (see the
git-reflog(1) mention). For instance, the ref/reference discussion can
be called a “see also” since the glossary here already goes into
detail. On the other hand, gitdatamodel(7) offers more details on
the subject of “the index”.

Let’s also add gitdatamodel(7) to See Also. It is at least as relevant
as the other tutorial pages that are already mentioned.

Signed-off-by: Kristoffer Haugsbakk <code@khaugsbakk.name>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/gitglossary.adoc

@@ -16,6 +16,7 @@ include::glossary-content.adoc[]
 
 SEE ALSO
 --------
+linkgit:gitdatamodel[7],
 linkgit:gittutorial[7],
 linkgit:gittutorial-2[7],
 linkgit:gitcvs-migration[7],

Documentation/glossary-content.adoc

@@ -251,7 +251,8 @@ for a more flexible and robust system to do the same thing.
 	as objects. The index is a stored version of your
 	<<def_working_tree,working tree>>. Truth be told, it can also contain a second, and even
 	a third version of a working tree, which are used
-	when <<def_merge,merging>>.
+	when <<def_merge,merging>>. See "THE INDEX" in
+	linkgit:gitdatamodel[7] for details.
 
 [[def_index_entry]]index entry::
 	The information regarding a particular file, stored in the
@@ -290,7 +291,8 @@ This commit is referred to as a "merge commit", or sometimes just a
 [[def_object]]object::
 	The unit of storage in Git. It is uniquely identified by the
 	<<def_SHA1,SHA-1>> of its contents. Consequently, an
-	object cannot be changed.
+	object cannot be changed. See "OBJECTS" in
+	linkgit:gitdatamodel[7] for details.
 
 [[def_object_database]]object database::
 	Stores a set of "objects", and an individual <<def_object,object>> is
@@ -587,12 +589,17 @@ extended in the future:
 Different subhierarchies are used for different purposes. For example,
 the `refs/heads/` hierarchy is used to represent local branches whereas
 the `refs/tags/` hierarchy is used to represent local tags..
++
+See also "REFERENCES" in linkgit:gitdatamodel[7].
 
 [[def_reflog]]reflog::
 	A reflog shows the local "history" of a ref.  In other words,
 	it can tell you what the 3rd last revision in _this_ repository
 	was, and what was the current state in _this_ repository,
-	yesterday 9:14pm.  See linkgit:git-reflog[1] for details.
+	yesterday 9:14pm.
++
+See "REFLOGS" in linkgit:gitdatamodel[7] for a short explanation of the
+format. See linkgit:git-reflog[1] for details.
 
 [[def_refspec]]refspec::
 	A "refspec" is used by <<def_fetch,fetch>> and