datetime_from_timet_and_us():  ignore leap seconds if the platform
localtime()/gmtime() insists on delivering them, + associated doc
changes.

Redid the docs for datetimtez.astimezone().
diff --git a/Misc/NEWS b/Misc/NEWS
index 89c7683..8bc29a6 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -50,6 +50,12 @@
   time), this case can arise one hour per year, at the hour daylight time
   ends.  See new docs for details.
 
+  The constructors building a datetime from a timestamp could raise
+  ValueError if the platform C localtime()/gmtime() inserted "leap
+  seconds".  Leap seconds are ignored now.  On such platforms, it's
+  possible to have timestamps that differ by a second, yet where
+  datetimes constructed from them are equal.
+
 Library
 -------