Documentation/git-checkout: Update summary to reflect current abilities

For a while now, git-checkout has been more powerful than the man-page
summary would suggest (the main text does describe the new features),
so update the summary to hopefully better reflect the current
functionality.  Also update the glossary description of the word checkout.

Signed-off-by: Julian Phillips <julian@quantumfyre.co.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-checkout.txt

@@ -3,7 +3,7 @@ git-checkout(1)
 
 NAME
 ----
-git-checkout - Checkout and switch to a branch
+git-checkout - Checkout a branch or paths to the working tree
 
 SYNOPSIS
 --------

Documentation/glossary.txt

@@ -45,9 +45,12 @@ GIT Glossary
 	"changesets" with git.
 
 [[def_checkout]]checkout::
-	The action of updating the <<def_working_tree,working tree>> to a
-	<<def_revision,revision>> which was stored in the
-	<<def_object_database,object database>>.
+	The action of updating all or part of the
+	<<def_working_tree,working tree>> with a <<def_tree_object,tree object>>
+	or <<def_blob_object,blob>> from the
+	<<def_object_database,object database>>, and updating the
+	<<def_index,index>> and <<def_HEAD,HEAD>> if the whole working tree has
+	been pointed at a new <<def_branch,branch>>.
 
 [[def_cherry-picking]]cherry-picking::
 	In <<def_SCM,SCM>> jargon, "cherry pick" means to choose a subset of