docs: fix "git stash [push]" documentation

docs: fix "git stash [push]" documentation

Both the synopsis and explanation are incorrect and contradict each
other.
The synopsis claims "push" can only be omitted when you do not give any
options and arguments.
The explanation correctly claims that non-option arguments are not
allowed, except pathspec elements preceded by double hyphens.
But it also adds "-p" to the list of exceptions, even though it is an
option argument.

Signed-off-by: Quentin Bernet <quentin.bernet@bluewin.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>