docs: inline std::ffi::c_str types to std::ffi

Rustdoc has no way to show that an item is stable,
but only at a different path. `std::ffi::c_str::NulError` is
not stable, but `std::ffi::NulError` is.

To avoid marking these types as unstable when someone just
wants to follow a link from `CString`, inline them into their
stable paths.

library/std/src/ffi/mod.rs

@@ -179,19 +179,19 @@ pub use core::ffi::{
     c_ulong, c_ulonglong, c_ushort,
 };
 
-#[doc(no_inline)]
+#[doc(inline)]
 #[stable(feature = "cstr_from_bytes_until_nul", since = "1.69.0")]
 pub use self::c_str::FromBytesUntilNulError;
-#[doc(no_inline)]
+#[doc(inline)]
 #[stable(feature = "cstr_from_bytes", since = "1.10.0")]
 pub use self::c_str::FromBytesWithNulError;
-#[doc(no_inline)]
+#[doc(inline)]
 #[stable(feature = "cstring_from_vec_with_nul", since = "1.58.0")]
 pub use self::c_str::FromVecWithNulError;
-#[doc(no_inline)]
+#[doc(inline)]
 #[stable(feature = "cstring_into", since = "1.7.0")]
 pub use self::c_str::IntoStringError;
-#[doc(no_inline)]
+#[doc(inline)]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use self::c_str::NulError;
 #[doc(inline)]