#1411695: clarify behavior of xml.sax.utils.[un]escape.

Doc/library/xml.sax.utils.rst

@@ -21,7 +21,8 @@ or as base classes.
 
    You can escape other strings of data by passing a dictionary as the optional
    *entities* parameter.  The keys and values must all be strings; each key will be
-   replaced with its corresponding value.
+   replaced with its corresponding value.  The characters ``'&'``, ``'<'`` and
+   ``'>'`` are always escaped, even if *entities* is provided.
 
 
 .. function:: unescape(data[, entities])
@@ -30,7 +31,8 @@ or as base classes.
 
    You can unescape other strings of data by passing a dictionary as the optional
    *entities* parameter.  The keys and values must all be strings; each key will be
-   replaced with its corresponding value.
+   replaced with its corresponding value.  ``'&amp'``, ``'&lt;'``, and ``'&gt;'``
+   are always unescaped, even if *entities* is provided.
 
    .. versionadded:: 2.3