discardable async procs are now an error (#14176)

* add discard warning in manual

tests/async/tdiscardableproc.nim

@@ -0,0 +1,9 @@
+discard """
+  errmsg: "Cannot make async proc discardable. Futures have to be checked with `asyncCheck` instead of discarded"
+"""
+
+import async
+
+proc foo {.async, discardable.} = discard
+
+foo()

tests/vm/tslow_tables.nim

@@ -26,5 +26,5 @@ proc hop(): bool =
 
   echo "done"
 
-const r = hop()
+const r {.used.} = hop()