bug#37787: [PATCH] Update `special-mode' docstring.

[Hong Xu]

* lisp/simple.el (special-mode): Add more explanation for
`special-mode'.
---
 lisp/simple.el | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/lisp/simple.el b/lisp/simple.el
index e44b877931b6..2548a176f0a9 100644
--- a/lisp/simple.el
+++ b/lisp/simple.el
@@ -474,7 +474,9 @@ special-mode-map
 
 (put 'special-mode 'mode-class 'special)
 (define-derived-mode special-mode nil "Special"
-  "Parent major mode from which special major modes should inherit."
+  "Parent major mode from which special major modes should inherit.
+A special major mode is intended to view specially formatted data
+rather than files.  It is read-only and usually lightweight."
   (setq buffer-read-only t))
 
 ;; Making and deleting lines.
-- 
2.20.1