glossary: more precise definition of tree-ish (a.k.a. treeish)

A tree-ish isn't a ref.  Also, mention dereferencing, and that a
commit dereferences to a tree, to support gitrevisions(7) and
rev-parse's error messages.

Signed-off-by: Richard Hansen <rhansen@bbn.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/glossary-content.txt

@@ -499,9 +499,18 @@ should not be combined with other pathspec.
 	<<def_tree,tree>> is equivalent to a <<def_directory,directory>>.
 
 [[def_tree-ish]]tree-ish (also treeish)::
-	A <<def_ref,ref>> pointing to either a <<def_commit_object,commit
-	object>>, a <<def_tree_object,tree object>>, or a <<def_tag_object,tag
-	object>> pointing to a tag or commit or tree object.
+	A <<def_tree_object,tree object>> or an <<def_object,object>>
+	that can be recursively dereferenced to a tree object.
+	Dereferencing a <<def_commit_object,commit object>> yields the
+	tree object corresponding to the <<def_revision,revision>>'s
+	top <<def_directory,directory>>.
+	The following are all tree-ishes:
+	a <<def_commit-ish,commit-ish>>,
+	a tree object,
+	a <<def_tag_object,tag object>> that points to a tree object,
+	a tag object that points to a tag object that points to a tree
+	object,
+	etc.
 
 [[def_unmerged_index]]unmerged index::
 	An <<def_index,index>> which contains unmerged