Fix up documentation around no_std

1. Fix the sections in the book to have the correct signatures. I've
   also marked them as `ignore`; there's no way to set the `no_std`
   feature for libc, so it pulls in the stdlib, so this wasn't even
   testing the actual thing it was testing. Better to just ignore.
2. Correcting libcore's docs for factual inaccuracy, and add a note
   about language items.

Fixes #33677

1 files changed, 8 insertions, 5 deletions

diff --git a/src/libcore/lib.rs b/src/libcore/lib.rs
index e849369d647..0ad1f671f15 100644
--- a/src/libcore/lib.rs
+++ b/src/libcore/lib.rs
@@ -25,6 +25,8 @@
 //!
 //! # How to use the core library
 //!
+//! Please note that all of these details are currently not considered stable.
+//!
 // FIXME: Fill me in with more detail when the interface settles
 //! This library is built on the assumption of a few existing symbols:
 //!
@@ -34,11 +36,12 @@
 //!   These functions are often provided by the system libc, but can also be
 //!   provided by the [rlibc crate](https://crates.io/crates/rlibc).
 //!
-//! * `rust_begin_unwind` - This function takes three arguments, a
-//!   `fmt::Arguments`, a `&str`, and a `u32`. These three arguments dictate
-//!   the panic message, the file at which panic was invoked, and the line.
-//!   It is up to consumers of this core library to define this panic
-//!   function; it is only required to never return.
+//! * `rust_begin_panic` - This function takes three arguments, a
+//!   `fmt::Arguments`, a `&'static str`, and a `u32`. These three arguments
+//!   dictate the panic message, the file at which panic was invoked, and the
+//!   line. It is up to consumers of this core library to define this panic
+//!   function; it is only required to never return. This requires a `lang`
+//!   attribute named `panic_fmt`.
 
 // Since libcore defines many fundamental lang items, all tests live in a
 // separate crate, libcoretest, to avoid bizarre issues.