Mention the lack of "errno" on select.error

pep-3151.txt

@@ -883,6 +883,25 @@ select
 * epoll objects raise IOError;
 * kqueue objects raise both OSError and IOError.
 
+As a side-note, not deriving from ``EnvironmentError`` means ``select.error``
+does not get the useful ``errno`` attribute.  User code must check ``args[0]``
+instead::
+
+    >>> signal.alarm(1); select.select([], [], [])
+    0
+    Traceback (most recent call last):
+      File "<stdin>", line 1, in <module>
+    select.error: (4, 'Interrupted system call')
+    >>> e = sys.last_value
+    >>> e
+    error(4, 'Interrupted system call')
+    >>> e.errno == errno.EINTR
+    Traceback (most recent call last):
+      File "<stdin>", line 1, in <module>
+    AttributeError: 'error' object has no attribute 'errno'
+    >>> e.args[0] == errno.EINTR
+    True
+
 signal
 ''''''