Add documentation about for used as higher ranked trait bounds

[czipperz]

Resolves #55416

[rust-highfive]

r? @withoutboats

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[Centril]

Hmm... I think the whole paragraph needs a redo...
Can you itemize as a list and avoid starting each sentence with "for is used..."?

[Centril]

r? @Centril

[Centril]

Looks great; @bors r+ rollup

[bors]

📌 Commit 65b5e57 has been approved by Centril

[czipperz]

Hold on r- this I forgot to add the link @Centril

[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:0217f436:start=1553703346967536404,finish=1553703433218078155,duration=86250541751
$ 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
$ export GCP_CACHE_BUCKET=rust-lang-ci-cache
Setting environment variables from .travis.yml
---

[00:04:10] travis_fold:start:tidy
travis_time:start:tidy
tidy check
[00:04:10] tidy error: /checkout/src/libstd/keyword_docs.rs:355: line longer than 100 chars
[00:04:12] some tidy checks failed
[00:04:12] 
[00:04:12] 
[00:04:12] 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"
[00:04:12] 
[00:04:12] 
[00:04:12] failed to run: /checkout/obj/build/bootstrap/debug/bootstrap test src/tools/tidy
[00:04:12] Build completed unsuccessfully in 0:00:47
[00:04:12] Build completed unsuccessfully in 0:00:47
[00:04:12] make: *** [tidy] Error 1
[00:04:12] Makefile:67: recipe for target 'tidy' failed
The command "stamp sh -x -c "$RUN_SCRIPT"" exited with 2.
travis_time:start:02a42780
$ date && (curl -fs --head https://google.com | grep ^Date: | sed 's/Date: //g' || true)
Wed Mar 27 16:21:35 UTC 2019
---
travis_time:end:0df6dea4:start=1553703695905566073,finish=1553703695910283875,duration=4717802
travis_fold:end:after_failure.3
travis_fold:start:after_failure.4
travis_time:start:19673d00
$ 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:05f6256c
travis_time:start:05f6256c
$ 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:1cc63899
$ 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)

[Centril]

@bors r-

[czipperz]

Apparently this line is too long in either case. Thoughts?

[Centril]

@bors r+ rollup

[bors]

📌 Commit 2545250 has been approved by Centril