docs: adding a note on ICU processing to Ivy compatibility guide (#33480

)

PR Close #33480

aio/content/guide/ivy-compatibility.md

@@ -60,3 +60,5 @@ The following changes will be visible more rarely, as they mostly deal in edge c
 - It's no longer possible to overwrite lifecycle hooks with mocks on directive instances for testing (instead, modify the lifecycle hook on the directive type itself). 
 
 - Special injection tokens (e.g. `TemplateRef` or `ViewContainerRef`) return a new instance whenever they are requested (previously, instances of special tokens were shared if requested on the same node). This primarily affects tests that do identity comparison of these objects.
+
+- ICU parsing happens at runtime, so only text, HTML tags and text bindings are allowed inside ICU cases (previously, directives were also permitted inside ICUs).