Update CHANGELOG and docs

with change to ActiveSupport::Notifications::Instrumenter#instrument

activesupport/CHANGELOG.md

+*   Update `ActiveSupport::Notifications::Instrumenter#instrument to make
+    passing a block optional. This will let users use
+    `ActiveSupport::Notifications` messaging features outside of
+    instrumentation.
+
+    *Ali Ibrahim*
+
 *   Fix `Time#advance` to work with dates before 1001-03-07
 
     Before:

activesupport/lib/active_support/notifications/instrumenter.rb

@@ -13,9 +13,10 @@ def initialize(notifier)
         @notifier = notifier
       end
 
-      # Instrument the given block by measuring the time taken to execute it
-      # and publish it. Notice that events get sent even if an error occurs
-      # in the passed-in block.
+      # Given a block, instrument it by measuring the time taken to execute
+      # and publish it. Without a block, simply send a message via the
+      # notifier. Notice that events get sent even if an error occurs in the
+      # passed-in block.
       def instrument(name, payload = {})
         # some of the listeners might have state
         listeners_state = start name, payload