Documentation: clarify "working tree" definition

It is not necessarily obvious to a git novice what it means for a
filesystem tree to be equal to the HEAD.  Spell it out.

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

Author: jrn

Documentation/glossary-content.txt

@@ -456,6 +456,6 @@ This commit is referred to as a "merge commit", or sometimes just a
 	of 'A' is 'origin/B' sometimes we say "'A' is tracking 'origin/B'".
 
 [[def_working_tree]]working tree::
-	The tree of actual checked out files.  The working tree is
-	normally equal to the <<def_HEAD,HEAD>> plus any local changes
-	that you have made but not yet committed.
+	The tree of actual checked out files.  The working tree normally
+	contains the contents of the <<def_HEAD,HEAD>> commit's tree,
+	plus any local changes that you have made but not yet committed.