Merge branch 'ib/doc-push-default-simple' into jch

The documentation for `push.default = simple` has been clarified to
better explain its behavior, making it clear that it pushes the
current branch to a same-named branch on the remote, and detailing
the upstream requirements for centralized workflows.

* ib/doc-push-default-simple:
  doc: clarify push.default=simple behavior