build: add REPLACEME tag for version info in docs

Add a `REPLACEME` tag that should be used when introducing
docs for new features, so that they can be updated when releases
are made.

Ref: #6578
PR-URL: #6864
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Claudio Rodriguez <cjrodr@yahoo.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

Makefile

@@ -439,6 +439,10 @@ PACKAGEMAKER ?= /Developer/Applications/Utilities/PackageMaker.app/Contents/MacO
 PKGDIR=out/dist-osx
 
 release-only:
+	@if `grep -q REPLACEME doc/api/*.md`; then \
+		echo 'Please update Added: tags in the documentation first.' ; \
+		exit 1 ; \
+	fi
 	@if [ "$(shell git status --porcelain | egrep -v '^\?\? ')" = "" ]; then \
 		exit 0 ; \
 	else \