* ext/tk/sample/tktextio.rb: add comment

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@8120 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

ext/tk/sample/tktextio.rb

@@ -1,9 +1,18 @@
 #!/usr/bin/env ruby
 #
 #  sample class of handling I/O stream on a TkText widget
-#
 #                                               by Hidetoshi NAGAI
 #
+#  NOTE: TkTextIO supports 'character' (not 'byte') access only. 
+#        So, for example, TkTextIO#getc returns a character, TkTextIO#pos 
+#        means the character position, TkTextIO#read(size) counts by 
+#        characters, and so on.
+#        Of course, it is available to make TkTextIO class to suuport 
+#        'byte' access. However, it may break multi-byte characters. 
+#        and then, displayed string on the text widget may be garbled.
+#        I think that it is not good on the supposed situation of using 
+#        TkTextIO. 
+#
 require 'tk'
 
 class TkTextIO < TkText
@@ -50,7 +59,6 @@ class TkTextIO < TkText
       @open[:r] = true;  @open[:w] = true
       @txtpos.set('end - 1 char')
     end
-
   end
 
   def <<(obj)