NEWS: add entries for thread_sync.c changes

I'm slightly worried about some external code subclassing
ConditionVariable, Queue, and SizedQueue and relying on them
being Structs.  However, they only started being Structs with
Ruby 2.1, and were implemented in pure Ruby before that; so
hopefully nobody notices that implementation detail.

Also, note the Mutex change as it may affect program design
when space can be saved.

* NEWS: entries for [Feature #13552] and [Feature #13517]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@59385 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

NEWS

@@ -141,6 +141,9 @@ with all sufficient information, see the ChangeLog file or Redmine
 * Random.raw_seed renamed to become Random.urandom.  It is now
   applicable to non-seeding purposes due to [Bug #9569].
 
+* ConditionVariable, Queue and SizedQueue reimplemented for speed.
+  They no longer subclass Struct. [Feature #13552]
+
 === Stdlib compatibility issues (excluding feature bug fixes)
 
 * mathn.rb
@@ -157,6 +160,8 @@ with all sufficient information, see the ChangeLog file or Redmine
 
 * SecureRandom now prefers OS-provided sources than OpenSSL. [Bug #9569]
 
+* Mutex rewritten to be smaller and faster [Feature #13517]
+
 === Miscellaneous changes
 
 * Print backtrace and error message in reverse order if STDERR is unchanged and a tty.