README.EXT.ja: using commas and periods

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47039 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
make symbol conversion functions public

* include/ruby/encoding.h (rb_check_symbol_cstr): ditto.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47040 b2dd03c8-39d4-4d8f-98ff-823fe69b080e