cont.c: change fiber stack size doc [ci skip]

It's many years since we had tiny 4KB fiber stacks :<
point to the manpage instead for current, larger sizes.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@57205 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

cont.c

@@ -1102,8 +1102,9 @@ rb_cont_call(int argc, VALUE *argv, VALUE contval)
  *  the programmer and not the VM.
  *
  *  As opposed to other stackless light weight concurrency models, each fiber
- *  comes with a small 4KB stack. This enables the fiber to be paused from deeply
+ *  comes with a stack.  This enables the fiber to be paused from deeply
- *  nested function calls within the fiber block.
+ *  nested function calls within the fiber block.  See the ruby(1)
+ *  manpage to configure the size of the fiber stack(s).
  *
  *  When a fiber is created it will not run automatically. Rather it must
  *  be explicitly asked to run using the <code>Fiber#resume</code> method.