Improve the documentation of atomic::fence

[matthieu-m]

Attempt to "fix" two flaws of the current documentation:

1. The over-emphasis of fence - fence synchronization, relegating atomic - fence and fence - atomic synchronization to second fiddle.
2. The lack of explanation as to how to properly perform atomic - fence and fence - atomic synchronization.

It does so by first making it clear that there are 3 different ways to use an atomic fence, then presenting a full example for each usecase, noting the particular position of the fence with regard to the atomic operation, and rounding up with generic notes.

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[matthieu-m]

In this case, and the fence - atomic case below, it's unclear whether the arrow should come from/point to the fence instead.

The happens-before relationship established is between X and B, but it may be worth emphasizing the X - Y link which is crucial for the fence to work.

[rust-log-analyzer]

The job tidy failed! Check out the build log: (web) (plain enhanced) (plain)

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checking C++ file formatting
spellchecking files
building external tool typos from package typos-cli@1.34.0
finished building tool typos
error: `Aquire` should be `Acquire`
##[error]  --> library/core/src/sync/atomic.rs:4356:18
     |
4356 | /// (at least) [`Aquire`] ordering semantics.
     |                  ^^^^^^
     |
rerun tidy with `--extra-checks=spellcheck --bless` to fix typos
tidy [extra_checks]: checks with external tool 'typos' failed
tidy [extra_checks]: FAIL
tidy: The following check failed: extra_checks
Command `/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-tools-bin/rust-tidy /checkout /checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo /checkout/obj/build 4 /node/bin/npm --extra-checks=py,cpp,js,spellcheck` failed with exit code 1
Created at: src/bootstrap/src/core/build_steps/tool.rs:1549:23
Executed at: src/bootstrap/src/core/build_steps/test.rs:1280:29

Command has failed. Rerun with -v to see more details.
Bootstrap failed while executing `test src/tools/tidy tidyselftest --extra-checks=py,cpp,js,spellcheck`
Build completed unsuccessfully in 0:02:30
  local time: Sun Oct 19 15:23:39 UTC 2025
  network time: Sun, 19 Oct 2025 15:23:40 GMT
##[error]Process completed with exit code 1.