document that fork doesn't copy other threads

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@9571 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

process.c

@@ -1401,6 +1401,9 @@ rb_fork(status, chfunc, charg)
  *  of its children or use <code>Process.detach</code> to register
  *  disinterest in their status; otherwise, the operating system
  *  may accumulate zombie processes.
+ *
+ *  The thread calling fork is the only thread in the created child process.
+ *  fork doesn't copy other threads.
  */
 
 static VALUE