Improve commentary in frame.el

* lisp/frame.el: Improve commentary for display-* functions.
(Bug#35058)

lisp/frame.el

@@ -1691,6 +1691,14 @@ for FRAME."
 
 ;;;; Frame/display capabilities.
 
+;; These functions should make the features they test explicit in
+;; their names, so that when capabilities or the corresponding Emacs
+;; features change, it will be easy to find all the tests for such
+;; capabilities by a simple text search.  See more about the history
+;; and the intent of these functions in
+;; http://lists.gnu.org/archive/html/bug-gnu-emacs/2019-04/msg00004.html
+;; or in https://debbugs.gnu.org/cgi/bugreport.cgi?bug=35058#17.
+
 (declare-function msdos-mouse-p "dosfns.c")
 
 (defun display-mouse-p (&optional display)