better help message in make file

	@echo "  old        create old-style HTML doc pages in old dir"

	@echo "  fetch      fetch HTML and PDF files from LAMMPS web site"

	@echo "  epub       create ePUB format manual for e-book readers"

	@echo "  mobi       create MOBI format manual for e-book readers (e.g. Kindle)"

	@echo "  mobi       convert ePUB to MOBI format manual for e-book readers (e.g. Kindle)"

	@echo "                      (requires ebook-convert tool from calibre)"

	@echo "  clean      remove all intermediate RST files"

	@echo "  clean-all  reset the entire build environment"

	@echo "  txt2html   build txt2html tool"