Add note regarding writable PVs and msg_apply_changes

docs/cookbooks/devel/pseudovariables.md

@@ -19,6 +19,8 @@ Pseudo-Variables are implemented by various modules, most of them are
 provided by **pv** (if there is no special reference to a module, expect
 that the pseudo-variable is provided by **pv** module).
 
+**_NOTE: When using pseudo-variables in the configuration file that are writable, to change part of the message, applying them all at once can result in unintended side-effects. To ensure that the changes are applied to the message as intended, it is recommended to use the `msg_apply_changes()` function from the `textopsx` module after each change to the message. This function can help to ensure that the changes are applied correctly and that the message remains valid._ [See FAQ for more info.](../../tutorials/faq/main.md#sip-message-processing)**
+
 ## Pseudo-variables usage
 
 Pseudo-variables can be used with many modules, among them: