Clarify thread::park semantics

[matklad]

It took me quite some time to realize that the example is not actually racy, so let's clarify it? :-)

[rust-highfive]

r? @kennytm

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[rust-highfive]

The job x86_64-gnu-llvm-5.0 of your PR failed on Travis (raw log). Through arcane magic we have determined that the following fragments from the build log may contain information about the problem.

Click to expand the log.

[00:03:59] travis_fold:start:tidy
travis_time:start:tidy
tidy check
[00:04:00] tidy error: /checkout/src/libstd/thread/mod.rs:734: trailing whitespace
[00:04:01] some tidy checks failed
[00:04:01] 
[00:04:01] 
[00:04:01] command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0-tools-bin/tidy" "/checkout/src" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo" "--no-vendor" "--quiet"
[00:04:01] 
[00:04:01] 
[00:04:01] failed to run: /checkout/obj/build/bootstrap/debug/bootstrap test src/tools/tidy
[00:04:01] Build completed unsuccessfully in 0:00:52
[00:04:01] Build completed unsuccessfully in 0:00:52
[00:04:01] make: *** [tidy] Error 1
[00:04:01] Makefile:79: recipe for target 'tidy' failed

The command "stamp sh -x -c "$RUN_SCRIPT"" exited with 2.
travis_time:start:0330c33c
$ date && (curl -fs --head https://google.com | grep ^Date: | sed 's/Date: //g' || true)
---
travis_time:end:1f8ec4b8:start=1532689698229214569,finish=1532689698235635242,duration=6420673
travis_fold:end:after_failure.3
travis_fold:start:after_failure.4
travis_time:start:050c7600
$ ln -s . checkout && for CORE in obj/cores/core.*; do EXE=$(echo $CORE | sed 's|obj/cores/core\.[0-9]*\.!checkout!\(.*\)|\1|;y|!|/|'); if [ -f "$EXE" ]; then printf travis_fold":start:crashlog\n\033[31;1m%s\033[0m\n" "$CORE"; gdb -q -c "$CORE" "$EXE" -iex 'set auto-load off' -iex 'dir src/' -iex 'set sysroot .' -ex bt -ex q; echo travis_fold":"end:crashlog; fi; done || true
travis_fold:end:after_failure.4
travis_fold:start:after_failure.5
travis_time:start:00063e3a
travis_time:start:00063e3a
$ cat ./obj/build/x86_64-unknown-linux-gnu/native/asan/build/lib/asan/clang_rt.asan-dynamic-i386.vers || true
cat: ./obj/build/x86_64-unknown-linux-gnu/native/asan/build/lib/asan/clang_rt.asan-dynamic-i386.vers: No such file or directory
travis_fold:end:after_failure.5
travis_fold:start:after_failure.6
travis_time:start:2bfc8cbe
$ dmesg | grep -i kill

I'm a bot! I can only do what humans tell me to, so if this was not helpful or you have suggestions for improvements, please ping or otherwise contact @TimNN. (Feature Requests)

[kennytm]

@bors r+ rollup

[bors]

📌 Commit 5f87f78 has been approved by kennytm