Merge pull request rust-lang#419 from RalfJung/overflow

specify which integer overflows we mean

src/what-unsafe-does.md

@@ -71,7 +71,7 @@ Rust considers it "safe" to:
 * Deadlock
 * Have a [race condition][race]
 * Leak memory
-* Overflow integers
+* Overflow integers (with the built-in operators such as `+` etc.)
 * Abort the program
 * Delete the production database