* error.c: clarify a document of SignalException. Process.kill()
  doesn't have any guarantee when signal will be delivered.
  [Bug #7951] [ruby-core:52864]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39501 b2dd03c8-39d4-4d8f-98ff-823fe69b080e