Add safety comment.

src/unsafe/unsafe-functions.md

@@ -1,13 +1,15 @@
 # Calling Unsafe Functions
 
 A function or method can be marked `unsafe` if it has extra preconditions you
-must uphold:
+must uphold to avoid undefined behaviour:
 
 ```rust,editable
 fn main() {
     let emojis = "🗻∈🌏";
+
+    // Safe because the indices are in the correct order, within the bounds of
+    // the string slice, and lie on UTF-8 sequence boundaries.
     unsafe {
-        // Undefined behavior if indices do not lie on UTF-8 sequence boundaries.
         println!("{}", emojis.get_unchecked(0..4));
         println!("{}", emojis.get_unchecked(4..7));
         println!("{}", emojis.get_unchecked(7..11));