Revise E0133 description.

Tweaks some of the wording so it reads better.

src/librustc/diagnostics.rs

@@ -312,8 +312,8 @@ http://doc.rust-lang.org/reference.html#ffi-attributes
 E0133: r##"
 Using unsafe functionality, such as dereferencing raw pointers and calling
 functions via FFI or marked as unsafe, is potentially dangerous and disallowed
-by safety checks. As such, those safety checks can be temporarily relaxed by
-wrapping the unsafe instructions inside an `unsafe` block. For instance:
+by safety checks. These safety checks can be relaxed for a section of the code
+by wrapping the unsafe instructions with an `unsafe` block. For instance:
 
 ```
 unsafe fn f() { return; }