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

Author: scrabsha

library/core/src/intrinsics/mod.rs

@@ -6,6 +6,9 @@
 //! and <https://github.com/rust-lang/rust/blob/master/compiler/rustc_codegen_llvm/src/intrinsic.rs>,
 //! and for const evaluation in <https://github.com/rust-lang/rust/blob/master/compiler/rustc_const_eval/src/interpret/intrinsics.rs>.
 //!
+//! The functions in this module are implementation details of `core` and should
+//! not be used outside of the standard library.
+//!
 //! # Const intrinsics
 //!
 //! In order to make an intrinsic unstable usable at compile-time, copy the implementation from