add some rdoc

* add reference to spawn to fork
* add an example which runs ruby

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@41043 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

process.c

@@ -3462,6 +3462,9 @@ rb_fork_ruby(int *status)
  *  fork doesn't copy other threads.
  *
  *  If fork is not usable, Process.respond_to?(:fork) returns false.
+ *
+ *  Note that fork(2) is not avaiable on some platforms like Windows and NetBSD 4.
+ *  Therefore you should use spawn() instead of fork().
  */
 
 static VALUE
@@ -3819,6 +3822,9 @@ rb_f_system(int argc, VALUE *argv)
  *    pid = spawn("tar xf ruby-2.0.0-p195.tar.bz2")
  *    Process.wait pid
  *
+ *    pid = spawn(RbConfig.ruby, "-eputs'Hello, world!'")
+ *    Process.wait pid
+ *
  *  This method is similar to Kernel#system but it doesn't wait for the command
  *  to finish.
  *