fix manual to reflect reality for .nosideeffect (#16781)

tests/effects/tnosideeffect.nim

@@ -0,0 +1,24 @@
+block: # `.noSideEffect`
+  func foo(bar: proc(): int): int = bar()
+  var count = 0
+  proc fn1(): int = 1
+  proc fn2(): int = (count.inc; count)
+
+  template accept(body) =
+    doAssert compiles(block:
+      body)
+
+  template reject(body) =
+    doAssert not compiles(block:
+      body)
+
+  accept:
+    func fun1() = discard foo(fn1)
+  reject:
+    func fun1() = discard foo(fn2)
+
+  var foo2: type(foo) = foo
+  accept:
+    func main() = discard foo(fn1)
+  reject:
+    func main() = discard foo2(fn1)