Add example in docs for std::thread::panicking.

[frewsxcv]

No description provided.

[frewsxcv]

I'm not mentally attached in any way to the example I wrote, so if anyone has better ideas, I'm very open to changing mine. This was prompted because it wasn't obvious to me what contexts panicking would be used, but after adding an example with Drop, I hope it clears up some confusion for others in the future.

[steveklabnik]

---- thread::panicking_0 stdout ----
    thread 'thread::panicking_0' panicked at 'test executable failed:
a: dropped while not unwinding
b: dropped while unwinding

thread 'main' panicked at 'explicit panic', <anon>:26
note: Run with `RUST_BACKTRACE=1` for a backtrace.
', src/librustdoc/test.rs:312
note: Run with `RUST_BACKTRACE=1` for a backtrace.


failures:
    thread::panicking_0

test result: FAILED. 475 passed; 1 failed; 14 ignored; 0 measured

This test needs to be marked should_panic.

/cc @rust-lang/libs is this a good example? Seems okay to me, but I have never used this method.

[alexcrichton]

Seems reasonable to me!

[frewsxcv]

This test needs to be marked should_panic.

Done.

[steveklabnik]

@bors: r+ rollup

[bors]

📌 Commit 9944b22 has been approved by steveklabnik

[bors]

⌛ Testing commit 9944b22 with merge d06f1dc...

[bors]

⛄ The build was interrupted to prioritize another pull request.

[bors]

⚡ Previous build results for auto-linux-32cross-opt, auto-linux-64-cargotest, auto-linux-64-cross-armhf, auto-linux-64-cross-freebsd, auto-mac-64-opt-rustbuild, auto-mac-cross-ios-opt, auto-win-msvc-32-cross-opt, auto-win-msvc-64-cargotest are reusable. Rebuilding only auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-cross-armsf, auto-linux-64-cross-netbsd, auto-linux-64-debug-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-opt-mir, auto-linux-64-opt-rustbuild, auto-linux-64-x-android-t, auto-linux-cross-opt, auto-linux-musl-64-opt, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-win-gnu-32-nopt-t, auto-win-gnu-32-opt, auto-win-gnu-32-opt-rustbuild, auto-win-gnu-64-nopt-t, auto-win-gnu-64-opt, auto-win-msvc-32-opt, auto-win-msvc-64-opt, auto-win-msvc-64-opt-mir, auto-win-msvc-64-opt-rustbuild...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-32cross-opt, auto-linux-64-cargotest, auto-linux-64-cross-armhf, auto-linux-64-cross-armsf, auto-linux-64-cross-freebsd, auto-linux-64-cross-netbsd, auto-linux-64-debug-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-opt-mir, auto-linux-64-opt-rustbuild, auto-linux-64-x-android-t, auto-linux-cross-opt, auto-linux-musl-64-opt, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-mac-64-opt-rustbuild, auto-mac-cross-ios-opt, auto-win-gnu-32-nopt-t, auto-win-gnu-32-opt, auto-win-gnu-32-opt-rustbuild, auto-win-gnu-64-nopt-t, auto-win-gnu-64-opt, auto-win-msvc-32-cross-opt, auto-win-msvc-32-opt, auto-win-msvc-64-cargotest, auto-win-msvc-64-opt, auto-win-msvc-64-opt-mir, auto-win-msvc-64-opt-rustbuild