build: don't require processing docs for nightlies

Opt-out `nightly` and `next-nightly` from the documentation
requirement since these docs aren't meant to be published.

This fixes our nightly jobs in CI.

PR-URL: https://github.com/nodejs/node/pull/8325
Fixes: https://github.com/nodejs/build/issues/478
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James Snell <jasnell@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>

Makefile

@@ -467,7 +467,8 @@ PACKAGEMAKER ?= /Developer/Applications/Utilities/PackageMaker.app/Contents/MacO
 PKGDIR=out/dist-osx
 
 release-only:
-	@if `grep -q REPLACEME doc/api/*.md`; then \
+	@if [ "$(DISTTYPE)" != "nightly" ] && [ "$(DISTTYPE)" != "next-nightly" ] && \
+		`grep -q REPLACEME doc/api/*.md`; then \
 		echo 'Please update Added: tags in the documentation first.' ; \
 		exit 1 ; \
 	fi