Documentation/checkout: clarify description

git checkout can be used to switch branches and to retrieve files from
the index or an arbitrary tree.  Split the description into
subsections corresponding to each mode to make each use easier to
understand.

Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-checkout.txt

@@ -15,39 +15,41 @@ SYNOPSIS
 
 DESCRIPTION
 -----------
-Retrieves files from the index or specified tree and writes them
-to the working tree.
+Updates files in the working tree to match the version in the index
+or the specified tree.  If no paths are given, 'git checkout' will
+also update `HEAD` to set the specified branch as the current
+branch.
 
-'git checkout' [-b <new branch>] [<branch>]::
+'git checkout' [<branch>]::
+'git checkout' -b <new branch> [<start point>]::
 
-	When <paths> are not given, this command switches branches by
-	updating the index, working tree, and HEAD to reflect the
-	specified branch.
+	This form switches branches by 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.
+If `-b` is given, a new branch is created as if linkgit:git-branch[1]
+were called and then checked out; 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>...]::
+'git checkout' [--patch] [<tree-ish>] [--] <pathspec>...::
 
-	When <paths> or --patch are given, this command does *not* switch
+	When <paths> or `--patch` are given, 'git checkout' *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
+	the index file or from a named <tree-ish> (most often a commit).  In
 	this case, the `-b` and `--track` options are meaningless and giving
 	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
+The index may contain unmerged entries because of a previous 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.
-Using -f will ignore these unmerged entries.  The contents from a
+Using `-f` will ignore these unmerged entries.  The contents from a
 specific side of the merge can be checked out of the index by
-using --ours or --theirs.  With -m, changes made to the working tree
-file can be discarded to recreate the original conflicted merge result.
+using `--ours` or `--theirs`.  With `-m`, changes made to the working tree
+file can be discarded to re-create the original conflicted merge result.
 
 OPTIONS
 -------