Add an unstable book entry

Author: oli-obk

src/doc/unstable-book/src/language-features/intrinsics.md

@@ -2,13 +2,32 @@
 
 The tracking issue for this feature is: None.
 
-Intrinsics are never intended to be stable directly, but intrinsics are often
+Intrinsics are rarely intended to be stable directly, but are usually
 exported in some sort of stable manner. Prefer using the stable interfaces to
 the intrinsic directly when you can.
 
 ------------------------
 
 
+## Intrinsics with fallback logic
+
+Many intrinsics can be written in pure rust, albeit inefficiently or without supporting
+some features that only exist on some backends. Backends can simply not implement those
+intrinsics without causing any code miscompilations or failures to compile.
+
+```rust
+#![feature(core_intrinsics)]
+
+#[rustc_intrinsic(name_of_things)]
+fn foo() -> u32 {
+    42
+}
+```
+
+## Intrinsics without fallback logic
+
+These must be implemented by all backends.
+
 These are imported as if they were FFI functions, with the special
 `rust-intrinsic` ABI. For example, if one was in a freestanding
 context, but wished to be able to `transmute` between types, and