Rollup merge of #141109 - oconnor663:pipe_example, r=Mark-Simulacrum

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

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.

Author: matthiaskrgr