doc: update RTOS documentation with common interrupt assignment and memory barrier note

Update rtos.rst Overview to document common interrupt assignment (SysTimer for
tick, SysTimer Software Interrupt for task switch) used by all RTOS ports.

Add note about `__RWMB()` memory barriers placed after interrupt enable/disable
and CSR operations in all RTOS ports to ensure proper synchronization on SMP
systems.

Also update changelog to reflect these documentation changes.

Signed-off-by: Huaqi Fang <578567190@qq.com>

Author: fanghuaqi

doc/source/changelog.rst

@@ -192,6 +192,7 @@ This is release version of ``0.9.0`` of Nuclei SDK.
 
 * Documentation
 
+  - Update :ref:`design_rtos` to document common interrupt assignment for all RTOS ports (SysTimer for tick, SysTimer Software Interrupt for task switch) and memory barrier usage in RTOS ports
   - Fix typo ``reuqests`` to ``requires`` in ``app.rst``
   - Update ``rtos.rst`` to mention about recent bugfix for RTOS porting, and other RTOSes supported but not in Nuclei SDK
   - Update the expected output of ``cpuinfo`` in ``app.rst``

doc/source/design/rtos.rst

@@ -14,6 +14,11 @@ In Nuclei SDK, we have support four most-used RTOSes in the world,
 Our RTOS port require Nuclei ECLIC interrupt controller, please make sure
 your Nuclei CPU is configured with ECLIC present.
 
+All RTOS ports use the same interrupt assignment:
+
+- ``SysTimer Interrupt`` is used as the RTOS SysTick for periodic tick
+- ``SysTimer Software Interrupt`` is used to do task switch
+
 If you want to use RTOS in your application, you can choose one
 of the supported RTOSes.
 
@@ -24,6 +29,10 @@ of the supported RTOSes.
     - When you want to develop RTOS application in Nuclei SDK, please
       don't reconfigure ``SysTimer`` and ``SysTimer Software Interrupt``,
       since it is already used by RTOS portable code.
+    - In all RTOS ports, ``__RWMB()`` memory barriers are placed after interrupt
+      enable/disable and CSR operations to ensure the CSR write completes before
+      subsequent instructions execute. This is especially important for SMP systems
+      to prevent CPU/compiler reordering issues.
 
 .. _design_rtos_freertos: