doc: mention that intrinsics should not be called in user code
Intrinsic functions declared in `std::intrinsics` are an implementation detail and should not be called directly by the user. The compiler explicitly warns against their use in user code:
```
warning: the feature `core_intrinsics` is internal to the compiler or standard library
--> src/lib.rs:1:12
|
1 | #![feature(core_intrinsics)]
| ^^^^^^^^^^^^^^^
|
= note: using it is strongly discouraged
= note: `#[warn(internal_features)]` on by default
```
[**Playground link**]
This PR documents what the compiler warning says: these intrinsics should not be used in user code.
[**Playground link**]: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2024&gist=1c893b0698291f550bbdde0151fd221b
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