doc: clarify push.default=simple behavior

doc: clarify push.default=simple behavior

The documentation for the 'simple' push mode currently singles out
the centralized workflow, which can cause confusion about its
behavior in other scenarios, such as triangular workflows.

Clarify that 'simple' always pushes the current branch to a branch
of the same name, but only enforces the strict upstream tracking
requirement when pushing back to the same remote being pulled from.

Suggested-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Ivan Baluta <ivanbaluta.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>