Documentation: update git-clone man page with new behavior

Update git-clone man page to reflect recent changes
(--use-separate-remote default and use of .git/config instead of
remotes files), and rewrite introduction.
Signed-off-by: NJ. Bruce Fields <bfields@citi.umich.edu>
Signed-off-by: NJunio C Hamano <junkio@cox.net>

Documentation/git-clone.txt

@@ -16,22 +16,21 @@ SYNOPSIS
 
 DESCRIPTION
 -----------
-Clones a repository into a newly created directory.  All remote
-branch heads are copied under `$GIT_DIR/refs/heads/`, except
-that the remote `master` is also copied to `origin` branch.
 
-In addition, `$GIT_DIR/remotes/origin` file is set up to have
-this line:
+Clones a repository into a newly created directory, creates
+remote-tracking branches for each branch in the cloned repository
+(visible using `git branch -r`), and creates and checks out a master
+branch equal to the cloned repository's master branch.
 
-	Pull: master:origin
-
-This is to help the typical workflow of working off of the
-remote `master` branch.  Every time `git pull` without argument
-is run, the progress on the remote `master` branch is tracked by
-copying it into the local `origin` branch, and merged into the
-branch you are currently working on.  Remote branches other than
-`master` are also added there to be tracked.
+After the clone, a plain `git fetch` without arguments will update
+all the remote-tracking branches, and a `git pull` without
+arguments will in addition merge the remote master branch into the
+current branch.
 
+This default configuration is achieved by creating references to
+the remote branch heads under `$GIT_DIR/refs/remotes/origin` and
+by initializing `remote.origin.url` and `remote.origin.fetch`
+configuration variables.
 
 OPTIONS
 -------