Remove do ... while loops from the tests and docs.

src/test/compile-fail/while-constraints.rs

@@ -0,0 +1,10 @@
+// error-pattern:unsatisfied precondition constraint (for example, init(y
+fn main() {
+
+    let y: int = 42;
+    let x: int;
+    loop {
+        log(debug, y);
+        while true { while true { while true { x <- y; } } }
+    }
+}