Added help line for dev-docs target.

Makefile

@@ -126,7 +126,7 @@ help:
 	@echo '  install-xen      - build and install the Xen hypervisor'
 	@echo '  install-tools    - build and install the control tools'
 	@echo '  install-kernels  - build and install guest kernels'
-	@echo '  install-docs     - build and install documentation'
+	@echo '  install-docs     - build and install user documentation'
 	@echo ''
 	@echo 'Building targets:'
 	@echo '  dist             - build and install everything into local dist directory'
@@ -136,7 +136,8 @@ help:
 	@echo '  tools            - build and install tools'
 	@echo '  kernels          - build and install guest kernels'
 	@echo '  kbuild           - synonym for make kernels'
-	@echo '  docs             - build and install docs'
+	@echo '  docs             - build and install user documentation'
+	@echo '  dev-docs         - build developer-only documentation'
 	@echo ''
 	@echo 'Cleaning targets:'
 	@echo '  clean            - clean the Xen, tools and docs (but not'