Make temporary change of using _strptime for time.strptime permanent.

Flesh out docs to better explain time.strptime (closes bug #697990).
diff --git a/Misc/NEWS b/Misc/NEWS
index af190bd..76ec673 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -18,6 +18,9 @@
 
 - SSL no longer crashes the interpreter when the remote side disconnects.
 
+- time.strptime now exclusively uses the Python implementation
+  contained within the _strptime module.
+
 Library
 -------