Fix type error on XSLTProcessor::transformToDoc return value with SimpleXML

The return type is wrong. You can also use this method with SimpleXML.
In fact, PHP provides a way that even t

Fix type error on XSLTProcessor::transformToDoc return value with SimpleXML

The return type is wrong. You can also use this method with SimpleXML.
In fact, PHP provides a way that even third party libraries can hook
into its XML handling. Therefore, we cannot even use the
SimpleXML|DOMDocument|false union type as third party extensions may
extend the possibilities.

Broke in 8.1 in 1b35056a33.

Closes GH-12315.

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