put the new for loop macros under an experimental switch named 'forLoopMacros'

doc/manual.rst

@@ -5380,6 +5380,7 @@ type ``system.ForLoopStmt`` can rewrite the entirety of a ``for`` loop:
     :test: "nim c $1"
 
   import macros
+  {.experimental: "forLoopMacros".}
 
   macro enumerate(x: ForLoopStmt): untyped =
     expectKind x, nnkForStmt
@@ -5406,6 +5407,10 @@ type ``system.ForLoopStmt`` can rewrite the entirety of a ``for`` loop:
     echo a2, " ", b2
 
 
+Currently for loop macros must be enabled explicitly
+via ``{.experimental: "forLoopMacros".}``.
+
+
 Special Types
 =============