Go back to iso-2022-7bit for titdic-cnv.el again

* admin/notes/unicode: Mention this.
* lisp/international/titdic-cnv.el:
Go back to iso-2022-7bit for this file, since utf-8-emacs unified
characters that tsanq-quick-converter did not want unified.
Problem reported by Eli Zaretskii in:
https://lists.gnu.org/r/emacs-devel/2020-01/msg00156.html

Author: eggert

admin/notes/unicode

@@ -256,11 +256,19 @@ nontrivial changes to the build process.
 
 	etc/tutorials/TUTORIAL.ja
 
+ * iso-2022-7bit
+
+     This file contains multiple Chinese charsets, and converting it
+     to UTF-8 would lose the charset property and would change the
+     code's behavior.  Although this could be worked around by
+     propertizing the strings, that hasn't been done.
+
+	lisp/international/titdic-cnv.el
+
  * utf-8-emacs
 
      These files contain characters that cannot be encoded in UTF-8.
 
-	lisp/international/titdic-cnv.el
 	lisp/language/ethio-util.el
 	lisp/language/ethiopic.el
 	lisp/language/ind-util.el