Doc patch, further describe and-mask nature of the permission system

Hi,

The attached documentation patch further describes the
additive nature of the permission system.

This patch makes some sweeping statements. It needs
review by someone who knows whats in all the corners.
(I'm sure this would happen anyway, but it seems worth
mentioning.)

Paragraph structure is:

Describe permission semantics. Provide example of a common
error (unchanged from pre-existing text.) Generalize from
the example's lesson and provide correct process. Show
the semantics apply uniformly throughout Postgres.

Regards,

Karl <kop(at)meme(dot)com>
Free Software: "You don't pay back, you pay forward."
-- Robert A. Heinlein