Update note to include recommendation to use .as_slice()

Author: JessRudder

src/libstd/primitive_docs.rs

@@ -387,7 +387,8 @@ mod prim_slice { }
 /// [`len()`]: #method.len
 ///
 /// Note: This example shows the internals of `&str`. `unsafe` should not be
-/// used to get a string slice under normal circumstances.
+/// used to get a string slice under normal circumstances. Use `.as_slice()`
+/// instead.
 mod prim_str { }
 
 #[doc(primitive = "tuple")]