Issue #17902: Clarify doc of ElementTree.iterparse

Doc/library/xml.etree.elementtree.rst

@@ -384,7 +384,8 @@ Functions
    and ``"end-ns"`` (the "ns" events are used to get detailed namespace
    information).  If *events* is omitted, only ``"end"`` events are reported.
    *parser* is an optional parser instance.  If not given, the standard
-   :class:`XMLParser` parser is used.  Returns an :term:`iterator` providing
+   :class:`XMLParser` parser is used.  *parser* can only use the default
+   :class:`TreeBuilder` as a target.  Returns an :term:`iterator` providing
    ``(event, elem)`` pairs.
 
    .. note::