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.dkSigned-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20191121205929.40371-2-keescook@chromium.orgSigned-off-by: Jonathan Corbet <corbet@lwn.net>

scripts/jobserver-count

@@ -24,6 +24,8 @@ try:
 	flags = os.environ['MAKEFLAGS']
 
 	# Look for "--jobserver=R,W"
+	# Note that GNU Make has used --jobserver-fds and --jobserver-auth
+	# so this handles all of them.
 	opts = [x for x in flags.split(" ") if x.startswith("--jobserver")]
 
 	# Parse out R,W file descriptor numbers and set them nonblocking.
@@ -53,6 +55,7 @@ os.write(writer, jobs)
 # If the jobserver was (impossibly) full or communication failed, use default.
 if len(jobs) < 1:
 	print(default)
+	sys.exit(0)
 
 # Report available slots (with a bump for our caller's reserveration).
 print(len(jobs) + 1)