do not track 'raise Defect' in the .raises: [] clause anymore (#14298)

* do not track 'raise Defect' in the .raises: [] clause anymore

* --panics:on maps 'raise Defect' to an unrecoverable fatal error

* make tests green again

* update the documentation too

tests/effects/teffects7.nim

@@ -1,5 +1,5 @@
 discard """
-  errormsg: "can raise an unlisted exception: ref FloatingPointDefect"
+  errormsg: "can raise an unlisted exception: ref ValueError"
   line: 10
 """
 
@@ -7,7 +7,7 @@ proc foo() {.raises: [].} =
   try:
     discard
   except KeyError:
-    raise newException(FloatingPointDefect, "foo")
+    raise newException(ValueError, "foo")
   except Exception:
     discard