Clarify some comments and messages

Co-authored-by: Daira Hopwood <daira@jacaranda.org>

halo2_proofs/src/dev.rs

@@ -809,7 +809,7 @@ impl<F: FieldExt> MockProver<F> {
     /// Any verification failures will be pretty-printed to stderr before the function
     /// panics.
     ///
-    /// This method is equivalent to:
+    /// Apart from the stderr output, this method is equivalent to:
     /// ```ignore
     /// assert_eq!(prover.verify(), Ok(()));
     /// ```

halo2_proofs/src/dev/failure.rs

@@ -37,7 +37,7 @@ impl fmt::Display for FailureLocation {
         match self {
             Self::InRegion { region, offset } => write!(f, "in {} at offset {}", region, offset),
             Self::OutsideRegion { row } => {
-                write!(f, "on row {}", row)
+                write!(f, "outside any region, on row {}", row)
             }
         }
     }