merge revision(s) 39501,39502:

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

	* error.c: clarify reason for sleep in SignalException example


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_0_0@39707 b2dd03c8-39d4-4d8f-98ff-823fe69b080e