glossary: a revision is just a commit

The current definition of 'revision' sounds like it is saying that a
revision is a tree object.  In reality it is just a commit.

This should be especially useful for people used to other revision
control systems trying to see how familiar concepts translate into git
terms.

Reported-by: Ramkumar Ramachandra <artagnon@gmail.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/glossary-content.txt

@@ -430,9 +430,7 @@ should not be combined with other pathspec.
 	<<def_merge,merge>> left behind.
 
 [[def_revision]]revision::
-	A particular state of files and directories which was stored in the
-	<<def_object_database,object database>>. It is referenced by a
-	<<def_commit_object,commit object>>.
+	Synonym for <<def_commit,commit>> (the noun).
 
 [[def_rewind]]rewind::
 	To throw away part of the development, i.e. to assign the