update documentation on 'knownissue' testcase flag

doc/testcases.txt

@@ -96,10 +96,10 @@ metadata keys:
 * ``custom``: if true, some implementation dependent features are tested,
   and comparison to other Ecmascript engines is not relevant.
 
-* ``knownbug``: if true, the test is categorized as failure but is shown
-  as a known bug which does not need immediate fixing (i.e. they are not
-  release blockers).  Bugs marked as known bugs may be difficult to fix
-  or may be low priority.
+* ``knownissue``: if true, the test is categorized as failure but is shown
+  as a known issue which does not need immediate fixing (i.e. they are not
+  release blockers).  Bugs marked as known issues may be difficult to fix,
+  may be low priority, or it may uncertain what the desired behavior is.
 
 Practices
 ---------