Display raw bytes as belonging to 'eight-bit' charset

* lisp/descr-text.el (describe-char):
* lisp/simple.el (what-cursor-position): Display characters in the
range #x3FFF80..#x3FFF9F as belonging to charset 'eight-bit', not
'tis620-2533'.
* lisp/international/mule-diag.el (describe-character-set):
Improve description of :supplementary-p.

lisp/descr-text.el

@@ -404,6 +404,12 @@ relevant to POS."
            (charset (if eight-bit-p 'eight-bit
                       (or (get-text-property pos 'charset)
                           (char-charset char))))
+           ;; TIS620.2533 overlaps eight-bit-control, but we want to
+           ;; show eight-bit for raw bytes, not some obscure character
+           ;; set no one heard of.
+           (charset (if (eq charset 'tis620-2533)
+                        'eight-bit
+                      charset))
            (composition (find-composition pos nil nil t))
            (component-chars nil)
            (display-table (or (window-display-table)

lisp/international/mule-diag.el

@@ -355,7 +355,8 @@ meanings of these arguments."
 		     (:iso-revision-number "ISO revision number: "
 					   number-to-string)
 		     (:supplementary-p
-		      "Used only as a parent of some other charset." nil)))
+		      "Used only as a parent or a subset of some other charset,
+or provided just for backward compatibility." nil)))
 	(let ((val (get-charset-property charset (car elt))))
 	  (when val
 	    (if (cadr elt) (insert (cadr elt)))

lisp/simple.el

@@ -1353,7 +1353,7 @@ in *Help* buffer.  See also the command `describe-char'."
 	(if (or (not coding)
 		(eq (coding-system-type coding) t))
 	    (setq coding (default-value 'buffer-file-coding-system)))
-	(if (eq (char-charset char) 'eight-bit)
+	(if (and (>= char #x3fff80) (<= char #x3fffff))
 	    (setq encoding-msg
 		  (format "(%d, #o%o, #x%x, raw-byte)" char char char))
 	  ;; Check if the character is displayed with some `display'