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 called outside the standard library.

[**Playground link**]: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2024&gist=1c893b0698291f550bbdde0151fd221b

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