[DOCS] Mention union types in ViewHelper docs (#6407)

* [DOCS] Mention union types in ViewHelper docs

Releases: main

* [TASK] Language checks

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Co-authored-by: Sarah McCarthy <sarahmccarthy123@yahoo.com>

Author: s2b

Documentation/ApiOverview/Fluid/DevelopCustomViewhelper.rst

@@ -115,8 +115,13 @@ through the array :php:`$this->arguments`, in method :php:`render()`.
 
 ..  tip::
 
-    Sometimes arguments can take various types. In this case, the type `mixed`
-    should be used.
+    If arguments can take various types, use a union type, for example
+    `array|string`. If any type should be allowed, use `mixed`
+    instead.
+
+.. versionadded:: 14.0
+
+    Union types for ViewHelper arguments are available since TYPO3 14.0.
 
 `render()`
 ----------