Add more description about Shift_JIS and CP50220.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@34623 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

enc/iso_2022_jp.h

@@ -5,7 +5,12 @@ ENC_ALIAS("ISO2022-JP", "ISO-2022-JP");
 ENC_REPLICATE("ISO-2022-JP-2", "ISO-2022-JP");
 ENC_ALIAS("ISO2022-JP2", "ISO-2022-JP-2");
 
-/* Windows Codepage 50220
+/*
+ * Name: CP50220
+ * MIBenum: 2260
+ * Link: http://www.iana.org/assignments/charset-reg/CP50220
+ *
+ * Windows Codepage 50220
  * a ISO-2022-JP variant.
  * This includes
  * * US-ASCII

enc/shift_jis.c

@@ -381,6 +381,10 @@ OnigEncodingDefine(shift_jis, Shift_JIS) = {
  * MIBenum: 17
  * Link: http://www.iana.org/assignments/character-sets
  * Link: http://ja.wikipedia.org/wiki/Shift_JIS
+ *
+ * Note that this Shift_JIS's 7bit part is US-ASCII not JIX X 0201
+ * because Shift_JIS must be ASCII compatible encoding.
+ * See also the conversion table (enc/trans/japanese_sjis.trans).
  */
 
 /*