Add the source code version number to documentation

Signed-off-by: Shawn O. Pearce <sop@google.com>
diff --git a/Documentation/Makefile b/Documentation/Makefile
index 1fd880d..89d2e51 100644
--- a/Documentation/Makefile
+++ b/Documentation/Makefile
@@ -53,7 +53,7 @@
 $(DOC_HTML): %.html : %.txt
 	rm -f $@+ $@
 	$(ASCIIDOC) \
-		-a toc \
+		-a toc -a 'revision=$(DOC_VERS)' \
 		-b xhtml11 -f asciidoc.conf \
 		$(ASCIIDOC_EXTRA) -o $@+ $<
 	mv $@+ $@