fix test; improve comment

lib/pure/sugar.nim

@@ -243,9 +243,11 @@ when defined(nimHasAlias):
   proc aliasImpl[T1, T2](name: T1, expr: T2) {.magic: "Alias".}
 
   template `:=`*(name, expr) =
-    ## Declares `a` as alias of `expr`, which must resolve to a symbol.
+    ## Declares `name` as alias of `expr`, which must resolve to a symbol.
+    ## Works with any symbol, e.g. iterator, template, macro, module, proc etc.
     runnableExamples:
-      echo2:=system.echo
+      {.push experimental: "aliasSym".}
+      echo2:=echo
       echo2 "hello"
       declared2:=system.declared
       doAssert declared2(echo2)