better help message in make file

doc/Makefile

@@ -42,7 +42,8 @@ help:
 	@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"