Updated docs regarding attributes updates in window covering cluster. (…

…#18840)

* Updated docs regarding attributes updates in window covering cluster.

The window-covering-server is missing information about necessity of
calling PostAttributeChange to update cluster attributes in case
of overriding MatterWindowCoveringClusterServerAttributeChangedCallback
method.

Added appropriate documentation.

* Update src/app/clusters/window-covering-server/window-covering-server.cpp

Co-authored-by: Boris Zbarsky <bzbarsky@apple.com>

* Update src/app/clusters/window-covering-server/window-covering-server.h

Co-authored-by: Boris Zbarsky <bzbarsky@apple.com>

Co-authored-by: Boris Zbarsky <bzbarsky@apple.com>

src/app/clusters/window-covering-server/window-covering-server.cpp

@@ -947,6 +947,11 @@ bool emberAfWindowCoveringClusterGoToTiltPercentageCallback(app::CommandHandler
 
 /**
  * @brief Cluster Attribute Changed Callback
+ *
+ * The method is implemented by default as a weak function and it takes care of updating
+ * the server attribute values by calling the PostAttributeChange method. If the application overrides
+ * this method, it needs to handle updating attributes (ideally by calling PostAttributeChange).
+ *
  */
 void __attribute__((weak))
 MatterWindowCoveringClusterServerAttributeChangedCallback(const app::ConcreteAttributePath & attributePath)

src/app/clusters/window-covering-server/window-covering-server.h

@@ -140,7 +140,11 @@ void TiltPositionSet(chip::EndpointId endpoint, NPercent100ths position);
 EmberAfStatus GetMotionLockStatus(chip::EndpointId endpoint);
 
 /**
- * @brief PostAttributeChange is called when an Attribute is modified
+ * @brief PostAttributeChange is called when an Attribute is modified.
+ *
+ * The method is called by MatterWindowCoveringClusterServerAttributeChangedCallback
+ * to update cluster attributes values. If the application overrides MatterWindowCoveringClusterServerAttributeChangedCallback,
+ * it should call the PostAttributeChange on its own.
  *
  * @param[in] endpoint
  * @param[in] attributeId