Mentioning the role of status in exceptions closes #3977

doc/Language/exceptions.pod6

@@ -66,10 +66,12 @@ default.
 
 A C<CATCH> block uses smartmatching similar to how C<given/when>
 smartmatches on options, thus it's possible to catch and handle various
-categories of exceptions inside a C<when> block.
+categories of exceptions inside a C<when> block. And it does so because,
+within the block, C<$_> is set to the exception that has been raised.
 
 To handle all exceptions, use a C<default> statement. This example prints out
-almost the same information as the normal backtrace printer.
+almost the same information as the normal backtrace printer; the I<dot>
+methods apply to C<$_>, which holds the C<Exception> within the C<CATCH> block.
 
     CATCH {
          default {