Documentation/checkout: clarify description

To the first-time reader, it may not be obvious that ‘git checkout’
has two modes, nor that if no branch is specified it will read
from the index.

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

Documentation/git-checkout.txt

@@ -15,17 +15,23 @@ SYNOPSIS
 
 DESCRIPTION
 -----------
+Retrieves files from the index or specified tree and writes them
+to the working tree.
+
+'git checkout' [-b <new branch>] [<branch>]::
 
 	When <paths> are not given, this command switches branches by
-updating the index, working tree, and HEAD to reflect the specified
-branch.
-
+	updating the index, working tree, and HEAD to reflect the
+	specified branch.
++
 If `-b` is given, a new branch is created and checked out, as if
 linkgit:git-branch[1] were called; in this case you can
 use the --track or --no-track options, which will be passed to `git
 branch`.  As a convenience, --track without `-b` implies branch
 creation; see the description of --track below.
 
+'git checkout' [--patch] [<tree-ish>] [--] [<pathspec>...]::
+
 	When <paths> or --patch are given, this command does *not* switch
 	branches.  It updates the named paths in the working tree from
 	the index file, or from a named <tree-ish> (most often a commit).  In
@@ -34,7 +40,7 @@ either of them results in an error. The <tree-ish> argument can be
 	used to specify a specific tree-ish (i.e. commit, tag or tree)
 	to update the index for the given paths before updating the
 	working tree.
-
++
 The index may contain unmerged entries after a failed merge.  By
 default, if you try to check out such an entry from the index, the
 checkout operation will fail and nothing will be checked out.