Add some links to the cell docs.

[ehuss]

This adds a few links to the cell module docs to make it a little easier to navigate to the types and functions it references.

[rust-highfive]

r? @joshtriplett

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[jfrimmel]

Suggested change

	//! [`Arc<T>`]: ../../std/sync/struct.Arc.html
	//! [`Rc<T>`]: ../../std/rc/struct.Rc.html
	//! [`RwLock<T>`]: ../../std/sync/struct.RwLock.html
	//! [`Mutex<T>`]: ../../std/sync/struct.Mutex.html
	//! [`atomic`]: ../../core/sync/atomic/index.html
	//! [`Arc<T>`]: crate::sync::Arc
	//! [`Rc<T>`]: crate::rc::Rc
	//! [`RwLock<T>`]: crate::sync::RwLock
	//! [`Mutex<T>`]: crate::sync::Mutex
	//! [`atomic`]: crate::sync::atomic

I think its time for using intra-doc-links should be used.

[ehuss]

I don't think that is possible. These docs live in core, but most of those only exist in std or alloc. I do not believe intra-doc-links support that.

[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: 
---
skip untracked path cpu-usage.csv during rustfmt invocations
skip untracked path src/doc/book/ during rustfmt invocations
skip untracked path src/doc/rust-by-example/ during rustfmt invocations
skip untracked path src/llvm-project/ during rustfmt invocations
Diff in /checkout/library/core/src/cell.rs at line 190:
 //! [`Mutex<T>`]: ../../std/sync/struct.Mutex.html
 //! [`atomic`]: ../../core/sync/atomic/index.html
-
-
 #![stable(feature = "rust1", since = "1.0.0")]
 use crate::cmp::Ordering;
 use crate::cmp::Ordering;
Running `"/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/rustfmt" "--config-path" "/checkout" "--edition" "2018" "--unstable-features" "--skip-children" "--check" "/checkout/library/core/src/cell.rs"` failed.
If you're running `tidy`, try again with `--bless`. Or, if you just want to format code, run `./x.py fmt` instead.
Build completed unsuccessfully in 0:00:22

[jackh726]

@bors r+ rollup

r? @jackh726

[bors]

📌 Commit 4749ad0 has been approved by jackh726