Document 'is test-assertion'

doc/Type/Routine.pod6

@@ -407,6 +407,22 @@ sub þor() is raw {
 þor()[1] = 'pantuflo';
 say @zipi;  # OUTPUT: «[zape pantuflo]␤»
 
+=head2 trait is test-assertion
+
+    multi sub trait_mod:<is>(Routine:D, :$test-assertion!)
+
+Declares that a routine generates test output (aka TAP). When
+failures are reported, the calling routine's location is used instead
+of this routine. For example:
+
+=begin code
+    use Test;
+    sub foo-test($value) is test-assertion {
+        is $value, 42, "is the value 42?";
+    }
+    foo-test(666);    # <-- error is reported on this line
+=end code
+
 =end pod
 
 # vim: expandtab softtabstop=4 shiftwidth=4 ft=perl6