bpo-39560: Document PyUnicode_FromKindAndData() kind transformation

Doc/c-api/unicode.rst

@@ -422,6 +422,12 @@ APIs:
    :c:func:`PyUnicode_KIND`).  The *buffer* must point to an array of *size*
    units of 1, 2 or 4 bytes per character, as given by the kind.
 
+   If necessary, the input *buffer* is copied and transformed into the
+   canonical representation.  For example, if the *buffer* is a UCS4 string
+   (:c:macro:`PyUnicode_4BYTE_KIND`) and it consists only of codepoints in
+   the UCS1 range, it will be transformed into UCS1
+   (:c:macro:`PyUnicode_1BYTE_KIND`).
+
    .. versionadded:: 3.3