doc: clarify security pre-release end of embargo date

doc: clarify security pre-release end of embargo date

Clarify that a fixed date will be used for end of embargo (public
disclosure) date while communicating with downstream stakeholders.

Initial document got a review that it gives an impression that
communicated embargo date can be a range like 'less than a week' which
is not the case. The range applies when defining the end of the embargo
date but a fix date will be communicated.

Signed-off-by: Ferruh Yigit <ferruh.yigit@intel.com>
Acked-by: John McNamara <john.mcnamara@intel.com>