doc: stdout blocking or non-blocking behaviour

Makes clear that the behaviour of stdout is blocking in Linux/Unix even when they refer to pipes.

The current documentation can lead to misunderstandings telling that stdout is always non-blocking (when it refers to a pipe). This is not the behaviour in linux/unix, because of the current working of uv_pipe_open in libuv.

There is a working in progress right now to change that behaviour, but it is not clear yet the direction of those changes. Meanwhile I propose to update the current documentation to tell the actual and current behaviour.

I commented details of that issue #3584 (https://github.com/joyent/node/issues/3584#issuecomment-34908285), in a old commit of libuv joyent/libuv@8c9cbee1b1fb19786405bdd92af5edfdab4cdbbe and in the node group https://groups.google.com/d/msg/nodejs/Ua4nmiNPZXY/Vq3pepnM9Q8J