Fix inconsistent Clone documentation.

[doitian]

Now, arrays of any size Clone if the element type is Clone. So remove the
the document that uses this as an example.

refs #57123

[rust-highfive]

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[doitian]

I'm not sure this is the best way to resolve the inconsistency. I cannot find out another example that the type is Copy but has to implement Clone manually.

However, simply removing the example leaves the section How can I implement Clone? strange. Maybe we need to add a simple comment such as

It is enough to derive both Copy and Clone in most conditions.

[doitian]

I found an example from #26925 , which has been added in this the PR to replace the original example.

[rust-highfive]

The job x86_64-gnu-llvm-6.0 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.

travis_time:end:08f3fb90:start=1545809205949295055,finish=1545809207014223774,duration=1064928719
$ git checkout -qf FETCH_HEAD
travis_fold:end:git.checkout

Encrypted environment variables have been removed for security reasons.
See https://docs.travis-ci.com/user/pull-requests/#pull-requests-and-security-restrictions
$ export SCCACHE_BUCKET=rust-lang-ci-sccache2
$ export SCCACHE_REGION=us-west-1
Setting environment variables from .travis.yml
$ export IMAGE=x86_64-gnu-llvm-6.0
---
[00:02:52] tidy error: /checkout/src/libcore/clone.rs:93: trailing whitespace
[00:02:54] some tidy checks failed
[00:02:54] 
[00:02:54] 
[00:02:54] 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"
[00:02:54] 
[00:02:54] 
[00:02:54] failed to run: /checkout/obj/build/bootstrap/debug/bootstrap test src/tools/tidy
[00:02:54] Build completed unsuccessfully in 0:00:46
[00:02:54] Build completed unsuccessfully in 0:00:46
[00:02:54] make: *** [tidy] Error 1
[00:02:54] Makefile:79: recipe for target 'tidy' failed
The command "stamp sh -x -c "$RUN_SCRIPT"" exited with 2.
travis_time:start:20d8d21c
$ date && (curl -fs --head https://google.com | grep ^Date: | sed 's/Date: //g' || true)
Wed Dec 26 07:29:50 UTC 2018
---
travis_time:end:27a0de38:start=1545809390464448156,finish=1545809390470681490,duration=6233334
travis_fold:end:after_failure.3
travis_fold:start:after_failure.4
travis_time:start:21334423
$ 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 --batch -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:04993f5f
travis_time:start:04993f5f
$ 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:091521a2
$ 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)

[tspiteri]

Compound words from quoted and non-quoted parts, like derived, look weird. (I realize it's part of the text that is not being changed here, and there is another instance just before the added example above.)

[bluss]

there doesn't seem to be a reason to markup derive like that IMO

[bluss]

@bors r+ rollup

Thanks!

[bors]

📌 Commit bbc8c93 has been approved by bluss

[bors]

⌛ Testing commit bbc8c93 with merge f35beb3...

[bors]

💔 Test failed - status-travis

[kennytm]

@bors retry

[bors]

⌛ Testing commit bbc8c93 with merge b2b7a06...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: bluss
Pushing b2b7a06 to master...