Simplify the documentation of the restrictions

Author: serhiy-storchaka

Doc/library/xml.etree.elementtree.rst

@@ -739,9 +739,8 @@ Functions
    *prefix* is a namespace prefix.  *uri* is a namespace uri.  Tags and
    attributes in this namespace will be serialized with the given prefix, if at
    all possible.
-   :exc:`ValueError` is raised if *prefix* is not a valid prefix,
-   is reserved (``xmlns`` and ``ns`` followed by digits),
-   or if *prefix* is ``xml`` and *uri* is not the XML namespace or vice versa.
+   :exc:`ValueError` is raised if *prefix* is invalid or reserved
+   (``ns`` followed by digits is reserved for the serializer).
 
    .. versionadded:: 3.2