iso_8859_2.c,iso_8859_6.c,iso_8859_7.c,iso_8859_8.c,iso_8859_9.c,
shift_jis.c,windows_1251.c}: add document about encodings.
* enc/cp949.c: divided into new file.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@15516 b2dd03c8-39d4-4d8f-98ff-823fe69b080e