Rephrase PyUnicode_CompareWithASCIIString() documentation

Doc/c-api/unicode.rst

@@ -1063,8 +1063,9 @@ They all return *NULL* or ``-1`` if an exception occurs.
 .. c:function:: int PyUnicode_CompareWithASCIIString(PyObject *uni, char *string)
 
    Compare a unicode object, *uni*, with *string* and return -1, 0, 1 for less
-   than, equal, and greater than, respectively. *string* is an ASCII-encoded
-   string (it is interpreted as ISO-8859-1).
+   than, equal, and greater than, respectively. It is best to pass only
+   ASCII-encoded strings, but the function interprets the input string as
+   ISO-8859-1 if it contains non-ASCII characters".
 
 
 .. c:function:: int PyUnicode_RichCompare(PyObject *left,  PyObject *right,  int op)