rdoc update.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@26394 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

process.c

@@ -2607,6 +2607,8 @@ rb_fork(int *status, int (*chfunc)(void*), void *charg, VALUE fds)
  *
  *  The thread calling fork is the only thread in the created child process.
  *  fork doesn't copy other threads.
+ *
+ *  If fork is not usable, Process.respond_to?(:fork) returns false.
  */
 
 static VALUE