child_process: describe how to open fd in child

clarify how the child end of a pipe should be opened.

discussion: https://groups.google.com/forum/#!topic/nodejs/F4m1K8MIqkw
You have typos in the above, link markdown that doesn't work, use of ChildProcess.stdin when you mean child's process.stdin, and describing a fd as being "opened", which is not correct.

But your essential point is valid, and the docs are terrible, I've addressed your specific concern and others in https://github.com/joyent/node/pull/8639