* README.EXT, README.EXT.ja: remove description of RARRAY_PTR()
  and add a caution of accessing internal data structure directly.
  Also add a description of rb_ary_store().
  [Bug #8399]



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43044 b2dd03c8-39d4-4d8f-98ff-823fe69b080e