Added a big-picture explanation for thread::park() & co.
As I said in https://www.reddit.com/r/rust/comments/4ihvv1/hey_rust_programmers_got_a_question_ask_here/d372s4i, the current explanation of the `park()` and `unpark()` is a bit unclear. It says that they're used for blocking, but then it goes on explaining the semantics in detail, leaving the bigger picture a bit unclear.
I added a short high-level explanation that explains how the functions are used. I also exposed the full paths (`thread::park()` and `thread::Thread::unpark()`), because `unpark()`, being a method, is not directly visible at the module level.
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