docs, parallelism: Fix failure path and add comment

docs, parallelism: Fix failure path and add comment

Rasmus noted that the failure path didn't correctly exit. Fix this and
add another comment about GNU Make's job server environment variable
names over time.

Reported-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Link: https://lore.kernel.org/lkml/eb25959a-9ec4-3530-2031-d9d716b40b20@rasmusvillemoes.dk
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20191121205929.40371-2-keescook@chromium.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>