docs-rst: Fix an warning when in interactive mode

When XeLaTeX is in interactive mode, it complains that
py@noticelength already exists. Rename it and declare it
only once to avoid such messages.

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