docs, parallelism: Do not leak blocking mode to other readers

docs, parallelism: Do not leak blocking mode to other readers

Setting non-blocking via a local copy of the jobserver file descriptor
is safer than just assuming other reader processes with the same fd open
are prepared for it to be non-blocking.

Suggested-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Link: https://lore.kernel.org/lkml/44c01043-ab24-b4de-6544-e8efd153e27a@rasmusvillemoes.dk
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20191121205929.40371-3-keescook@chromium.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>