Improve docs for Formatter

[steveklabnik]

Some improvements to std::fmt::Formatter to make it a bit more consistent with other documentation, as well as calling out that you don't ever instantiate one yourself.

[rust-highfive]

r? @Kimundi

(rust_highfive has picked a reviewer for you, use r? to override)

[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:10f11e97:start=1546980702489369539,finish=1546980703551545882,duration=1062176343
$ 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
---

[00:03:35] travis_fold:start:tidy
travis_time:start:tidy
tidy check
[00:03:35] tidy error: /checkout/src/libcore/fmt/mod.rs:236: trailing whitespace
[00:03:35] tidy error: /checkout/src/libcore/fmt/mod.rs:240: trailing whitespace
[00:03:35] tidy error: /checkout/src/libcore/fmt/mod.rs:244: trailing whitespace
[00:03:37] some tidy checks failed
[00:03:37] 
[00:03:37] 
[00:03:37] 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"
[00:03:37] 
[00:03:37] 
[00:03:37] failed to run: /checkout/obj/build/bootstrap/debug/bootstrap test src/tools/tidy
[00:03:37] Build completed unsuccessfully in 0:00:48
[00:03:37] Build completed unsuccessfully in 0:00:48
[00:03:37] make: *** [tidy] Error 1
[00:03:37] Makefile:69: recipe for target 'tidy' failed
The command "stamp sh -x -c "$RUN_SCRIPT"" exited with 2.
travis_time:start:05e9973b
$ date && (curl -fs --head https://google.com | grep ^Date: | sed 's/Date: //g' || true)
Tue Jan  8 20:55:31 UTC 2019
---
travis_time:end:068a0808:start=1546980932005699881,finish=1546980932012177283,duration=6477402
travis_fold:end:after_failure.3
travis_fold:start:after_failure.4
travis_time:start:1be36ffd
$ 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:06595b4f
travis_time:start:06595b4f
$ 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:1a7061a0
$ 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)

[steveklabnik]

maybe r? @frewsxcv

[frewsxcv]

✨

[frewsxcv]

@bors r+ rollup

[bors]

📌 Commit 5fc6fc3 has been approved by frewsxcv