discuss deadlocks in the std::io::pipe() example

[oconnor663]

I think it's important to discuss deadlocks in examples of how to use pipes. The current example does include an explicit drop(), but it also implicitly relies on the fact that the Command object is temporary, so that it drops its copy of pong_tx. This sort of thing tends to trip people up when they use pipes for the first time. I might've gone overboard with the comments in this version, but I'm curious what folks think.

[rustbot]

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