Clarify the storage argument. (#6849)

* Slightly more consistent indentation with other parts of the document.

NEWS.md

@@ -104,11 +104,11 @@ Note: We're only listing outstanding class updates.
 
 * Fiber
 
-    *   Introduce Fiber.[] and Fiber.[]= for inheritable fiber storage.
-        Introduce Fiber#storage and Fiber#storage= (experimental) for
-        getting and resetting the current storage.  Introduce
-        `Fiber.new(storage:)` for setting the storage when creating a
-        fiber. [[Feature #19078]]
+    * Introduce Fiber.[] and Fiber.[]= for inheritable fiber storage.
+      Introduce Fiber#storage and Fiber#storage= (experimental) for
+      getting and resetting the current storage.  Introduce
+      `Fiber.new(storage:)` for setting the storage when creating a
+      fiber. [[Feature #19078]]
 
         Existing Thread and Fiber local variables can be tricky to use.
         Thread local variables are shared between all fibers, making it

include/ruby/internal/intern/cont.h

@@ -39,14 +39,18 @@ RBIMPL_SYMBOL_EXPORT_BEGIN()
 VALUE rb_fiber_new(rb_block_call_func_t func, VALUE callback_obj);
 
 /**
- * Creates a Fiber instance from a C-backended block with the specified storage.
+ * Creates a Fiber instance from a C-backended block with the specified
+ * storage.
  *
- * If the given storage is Qundef, this function is equivalent to
+ * If the given storage is Qundef or Qtrue, this function is equivalent to
  * rb_fiber_new() which inherits storage from the current fiber.
  *
  * If the given storage is Qfalse, this function uses the current fiber's
  * storage by reference.
  *
+ * Specifying either Qtrue or Qfalse is experimental and may be changed in the
+ * future.
+ *
  * If the given storage is Qnil, this function will lazy initialize the
  * internal storage which starts of empty (without any inheritance).
  *
@@ -54,6 +58,7 @@ VALUE rb_fiber_new(rb_block_call_func_t func, VALUE callback_obj);
  *
  * @param[in]  func          A function, to become the fiber's body.
  * @param[in]  callback_obj  Passed as-is to `func`.
+ * @param[in]  storage       The way to set up the storage for the fiber.
  * @return     An allocated  new instance  of rb_cFiber, which  is ready  to be
  *             "resume"d.
  */