Improve _autoconf.py comments

This adds a docstring to the _autoconf.py file detailing how it's
generated (the other comment it's not visible in pydoc/epydoc).

Reviewed-by: amishchenko

Makefile.am

@@ -269,6 +269,15 @@ lib/_autoconf.py: Makefile stamp-directories
 	set -e; \
 	{ echo '# This file is automatically generated, do not edit!'; \
 	  echo '#'; \
+	  echo ''; \
+	  echo '"""Build-time configuration for Ganeti.'; \
+	  echo '';\
+	  echo 'This file is autogenerated by the build process.'; \
+	  echo 'For any changes you need to re-run ./configure (and'; \
+	  echo 'not edit by hand).'; \
+	  echo ''; \
+	  echo '"""'; \
+	  echo ''; \
 	  echo "PACKAGE_VERSION = '$(PACKAGE_VERSION)'"; \
 	  echo "VERSION_MAJOR = '$(VERSION_MAJOR)'"; \
 	  echo "VERSION_MINOR = '$(VERSION_MINOR)'"; \