docs-rst: pdf: use same vertical margin on all Sphinx versions

Currently, on Sphinx up to version 1.4, pdf output uses a vertical
margin of 1 inch. For upper versions, it uses a margin of 0.5 inches.

That causes both page headers and footers to be very close to the margin
of the sheet. Not all printers support writing like that.

Also, there's no reason why the layout for newer versions would be
different than for previous ones.

So, standardize it, by always setting to 1 inch.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>