doc: push: explain default=simple correctly

Now that the code has been simplified and it's clear what it's
actually doing, update the documentation to reflect that.

Namely; the simple mode only barfs when working on a centralized
workflow, and there's no configured upstream branch with the same name.

Cc: Elijah Newren <newren@gmail.com>
Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config/push.txt

@@ -24,15 +24,14 @@ push.default::
 
 * `tracking` - This is a deprecated synonym for `upstream`.
 
-* `simple` - in centralized workflow, work like `upstream` with an
-  added safety to refuse to push if the upstream branch's name is
-  different from the local one.
+* `simple` - pushes the current branch with the same name on the remote.
 +
-When pushing to a remote that is different from the remote you normally
-pull from, work as `current`.  This is the safest option and is suited
-for beginners.
+If you are working on a centralized workflow (pushing to the same repository you
+pull from, which is typically `origin`), then you need to configure an upstream
+branch with the same name.
 +
-This mode has become the default in Git 2.0.
+This mode is the default since Git 2.0, and is the safest option suited for
+beginners.
 
 * `matching` - push all branches having the same name on both ends.
   This makes the repository you are pushing to remember the set of