[RTEMS DOCS 5] task-stack-alloc.rst: Add CONFIGURE_TASK_STACK_FROM_ALLOCATOR

[Joel Sherrill]

Updates #4520.
---
 c-user/config/task-stack-alloc.rst | 39 +++++++++++++++++++++++++++++++++++++-
 1 file changed, 38 insertions(+), 1 deletion(-)

diff --git a/c-user/config/task-stack-alloc.rst b/c-user/config/task-stack-alloc.rst
index 297c624..054f301 100644
--- a/c-user/config/task-stack-alloc.rst
+++ b/c-user/config/task-stack-alloc.rst
@@ -1,7 +1,7 @@
 .. SPDX-License-Identifier: CC-BY-SA-4.0
 
 .. Copyright (C) 2020 embedded brains GmbH (http://www.embedded-brains.de)
-.. Copyright (C) 1988, 2008 On-Line Applications Research Corporation (OAR)
+.. Copyright (C) 1988, 2008, 2021 On-Line Applications Research Corporation (OAR)
 
 Task Stack Allocator Configuration
 ==================================
@@ -115,6 +115,43 @@ NOTES:
 
     * `CONFIGURE_TASK_STACK_DEALLOCATOR`
 
+.. index:: _CONFIGURE_TASK_STACK_ALLOCATOR_FOR_IDLE
+.. index:: IDLE task stack allocator
+
+.. _CONFIGURE_TASK_STACK_ALLOCATOR_FOR_IDLE:
+
+CONFIGURE_TASK_STACK_ALLOCATOR_FOR_IDLE
+---------------------------------------
+
+CONSTANT:
+    ``CONFIGURE_TASK_STACK_ALLOCATOR_FOR_IDLE``
+
+OPTION TYPE:
+    This configuration option is an initializer define.
+
+DEFAULT VALUE:
+    The default value is ``_Stack_Allocator_allocate_for_idle``, which
+    indicates that IDLE task stacks will be allocated from an area statically
+    reserved by `<rtems/confdefs.h>``.
+
+VALUE CONSTRAINTS:
+    The value of this configuration option shall be defined to a valid function
+    pointer of the type ``void *( *allocate )( Per_CPU_Control *, size_t )``.
+
+DESCRIPTION:
+    The value of this configuration option initializes the IDLE stack allocator
+    allocate handler.
+
+NOTES:
+    A correctly configured system shall only specify this configuration option
+    if the task stack allocators are configured and the following are consistent:
+
+    * :ref:`CONFIGURE_TASK_STACK_ALLOCATOR_INIT`
+
+    * :ref:`CONFIGURE_TASK_STACK_ALLOCATOR`
+
+    * :ref:`CONFIGURE_TASK_STACK_DEALLOCATOR`
+
 .. index:: CONFIGURE_TASK_STACK_FROM_ALLOCATOR
 .. index:: task stack allocator
 
-- 
1.8.3.1