Improve comments on ! tests

Author: canndrew

src/test/compile-fail/never-fallback.rs

@@ -30,6 +30,7 @@ impl Flah {
 }
 
 fn doit() -> Result<(), ()> {
+    // The type of _ is unconstrained here and should default to !
     let _ = try!(Flah.flah()); //~ ERROR the trait bound
     Ok(())
 }

src/test/run-pass/unit-fallback.rs

@@ -27,6 +27,7 @@ impl Flah {
 }
 
 fn doit() -> Result<(), ()> {
+    // The type of _ is unconstrained here and should default to ()
     let _ = try!(Flah.flah());
     Ok(())
 }