Add a note about the correctness and the effect on unsafe code to the ExactSizeIterator docs

[sdroege]

As it is a safe trait it does not provide any guarantee that the
returned length is correct and as such unsafe code must not rely on it.

That's why TrustedLen exists.

Fixes #81739

[rust-highfive]

r? @kennytm

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[rust-log-analyzer]

The job mingw-check failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

configure: rust.channel         := nightly
configure: rust.debug-assertions := True
configure: llvm.assertions      := True
configure: dist.missing-tools   := True
configure: build.configure-args := ['--enable-sccache', '--disable-manage-submodu ...
configure: writing `config.toml` in current directory
configure: 
configure: run `python /checkout/x.py --help`
configure: 
---
 Documenting core v0.0.0 (/checkout/library/core)
error: unresolved link to `core::iter::TrustedLen`
  --> library/core/src/iter/traits/exact_size.rs:19:26
   |
19 | /// unstable and unsafe [`core::iter::TrustedLen`] trait gives this additional
   |                          ^^^^^^^^^^^^^^^^^^^^^^^^ no item named `core` in scope
   |
   = note: `-D broken-intra-doc-links` implied by `-D warnings`
error: unresolved link to `core::iter::TrustedLen`
  --> library/core/src/iter/traits/exact_size.rs:19:26
   |
   |
19 | /// unstable and unsafe [`core::iter::TrustedLen`] trait gives this additional
   |                          ^^^^^^^^^^^^^^^^^^^^^^^^ no item named `core` in scope
error: aborting due to 2 previous errors

error: could not document `core`


Caused by:
  process didn't exit successfully: `/checkout/obj/build/bootstrap/debug/rustdoc --edition=2018 --crate-type lib --crate-name core library/core/src/lib.rs --target x86_64-unknown-linux-gnu -o /checkout/obj/build/x86_64-unknown-linux-gnu/stage0-std/x86_64-unknown-linux-gnu/doc --error-format=json --json=diagnostic-rendered-ansi --markdown-css rust.css --markdown-no-toc -Z unstable-options --resource-suffix 1.51.0 --index-page /checkout/src/doc/index.md -L dependency=/checkout/obj/build/x86_64-unknown-linux-gnu/stage0-std/x86_64-unknown-linux-gnu/release/deps -L dependency=/checkout/obj/build/x86_64-unknown-linux-gnu/stage0-std/release/deps --cfg=bootstrap -Dwarnings -Winvalid_codeblock_attributes --crate-version '1.51.0-nightly
  (3838eda0f
  2021-02-04)'` (exit code: 1)


command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo" "rustdoc" "--target" "x86_64-unknown-linux-gnu" "-Zbinary-dep-depinfo" "-j" "16" "--release" "--locked" "--color" "always" "--features" "panic-unwind backtrace compiler-builtins-c" "--manifest-path" "/checkout/library/test/Cargo.toml" "-p" "core" "--" "--markdown-css" "rust.css" "--markdown-no-toc" "-Z" "unstable-options" "--resource-suffix" "1.51.0" "--index-page" "/checkout/src/doc/index.md"


failed to run: /checkout/obj/build/bootstrap/debug/bootstrap doc --stage 0 library/std
Build completed unsuccessfully in 0:00:09

[kennytm]

@bors r+ rollup

[bors]

📌 Commit f436630 has been approved by kennytm