Document lower_intrinsics intrinsics

Author: oli-obk

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

@@ -24,6 +24,13 @@ fn foo() -> u32 {
 }
 ```
 
+## Intrinsics lowered to MIR instructions
+
+Various intrinsics have native MIR operations that they correspond to. Instead of requiring
+backends to implement both the intrinsic and the MIR operation, the `lower_intrinsics` pass
+will convert the calls to the MIR operation. Backends do not need to know about these intrinsics
+at all.
+
 ## Intrinsics without fallback logic
 
 These must be implemented by all backends.