Add more details to NEWS

Misc/NEWS.d/next/Library/2018-02-15-08-18-52.bpo-31333.4fF-gM.rst

@@ -1,3 +1,10 @@
-``_abc`` module is added.  It is speedup module implemented in C for
-``abc``. Creating ABC subclass and calling ``isinstance`` or ``issubclass``
-with ABC subclass are up to 1.5x faster.
+``_abc`` module is added.  It is a speedup module with C implementations for
+various functions and methods in ``abc``.  Creating an ABC subclass and calling
+``isinstance`` or ``issubclass`` with an ABC subclass are up to 1.5x faster.
+In addition, this makes Python start-up up to 10% faster.
+
+Note that the new implementation hides internal registry and caches, previously
+accessible via private attributes ``_abc_registry``, ``_abc_cache``, and
+``_abc_negative_cache``.  There are three debugging helper methods that can be
+used instead ``_dump_registry``, ``_abc_registry_clear``, and
+``_abc_caches_clear``.