Improve commentary in info.el

* lisp/info.el: Explain in commentary why some commands start with
"info-" and others with "Info-".  See also
http://lists.gnu.org/archive/html/emacs-devel/2017-11/msg00482.html.

lisp/info.el

@@ -30,6 +30,15 @@
 ;; This distinction is to support indexing of computer programming
 ;; language terms that may contain ":" but not ": ".
 
+;; The commands in this file should start either with "Info-" or with
+;; "info-".  The capitalized version is for commands that are bound to
+;; keys, and therefore are unlikely to be invoked by name via "M-x";
+;; the lower-case version is for commands invoked by name.  This
+;; arrangement makes completion of "info-" commands work better,
+;; because the "Info-" commands (of which there are a lot) don't get
+;; in the way.  Please adhere to this convention when you add commands
+;; here.
+
 ;;; Code:
 
 (eval-when-compile (require 'cl-lib))