doc/checkout: refer to git-branch(1) as appropriate

Most of description for the branch creation options is
simply cut and paste from git-branch. There are two reasons
to fix this:

  1. It can grow stale with respect to what's in "git
     branch" (which it is now is).

  2. It is not just an implementation detail, but rather the
     desired mental model for the command that we are using
     "git branch" here. Being explicit about that can help
     the user understand what is going on.

It also makes sense to strip the branch creation options
from the synopsis, as they are making it a long,
hard-to-read line. They are still easily discovered by
reading the options list, and --track is explicitly
referenced when branch creation is described.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-checkout.txt

@@ -8,7 +8,7 @@ git-checkout - Checkout a branch or paths to the working tree
 SYNOPSIS
 --------
 [verse]
-'git checkout' [-q] [-f] [-t | --track | --no-track] [-b <new_branch> [-l]] [-m] [<branch>]
+'git checkout' [-q] [-f] [-m] [-b <new_branch>] [<branch>]
 'git checkout' [-f|--ours|--theirs|-m|--conflict=<style>] [<tree-ish>] [--] <paths>...
 
 DESCRIPTION
@@ -18,8 +18,9 @@ When <paths> are not given, this command switches branches by
 updating the index and working tree to reflect the specified
 branch, <branch>, and updating HEAD to be <branch> or, if
 specified, <new_branch>.  Using -b will cause <new_branch> to
-be created; in this case you can use the --track or --no-track
-options, which will be passed to `git branch`.
+be created 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 will default to creating a branch whose
 name is constructed from the specified branch name by stripping
@@ -62,22 +63,12 @@ entries; instead, unmerged entries are ignored.
 
 -b::
 	Create a new branch named <new_branch> and start it at
-	<branch>.  The new branch name must pass all checks defined
-	by linkgit:git-check-ref-format[1].  Some of these checks
-	may restrict the characters allowed in a branch name.
+	<branch>; see linkgit:git-branch[1] for details.
 
 -t::
 --track::
-	When creating a new branch, set up configuration so that 'git-pull'
-	will automatically retrieve data from the start point, which must be
-	a branch. Use this if you always pull from the same upstream branch
-	into the new branch, and if you don't want to use "git pull
-	<repository> <refspec>" explicitly. This behavior is the default
-	when the start point is a remote branch. Set the
-	branch.autosetupmerge configuration variable to `false` if you want
-	'git checkout' and 'git branch' to always behave as if '--no-track' were
-	given. Set it to `always` if you want this behavior when the
-	start point is either a local or remote branch.
+	When creating a new branch, set up "upstream" configuration. See
+	"--track" in linkgit:git-branch[1] for details.
 +
 If no '-b' option is given, the name of the new branch will be
 derived from the remote branch.  If "remotes/" or "refs/remotes/"
@@ -94,9 +85,8 @@ explicitly give a name with '-b' in such a case.
 	branch.autosetupmerge configuration variable is true.
 
 -l::
-	Create the new branch's reflog.  This activates recording of
-	all changes made to the branch ref, enabling use of date
-	based sha1 expressions such as "<branchname>@\{yesterday}".
+	Create the new branch's reflog; see linkgit:git-branch[1] for
+	details.
 
 -m::
 --merge::