[CHANGELOG] Improve entry about `source` directive.

CHANGELOG.md

@@ -20,10 +20,14 @@ To install or update CocoaPods see this [guide](http://docs.cocoapods.org/guides
   should not be used. It will be removed in future versions of CocoaPods.  
   [#2449](https://github.com/CocoaPods/CocoaPods/issues/2449)
 
-* Add 'specific repo sources' support. Allows one to specify in their Podfile
-  which sources should be used to retrieve specs from and the priority order
-  thereof. For example: `source 'netbe'`, or `source 'master'`, with 'master'
-  being the default GitHub CocoaPods spec-repo.  
+* Add support for loading podspecs from specific spec-repos _only_, a.k.a. ‘sources’.
+  By default, when not specifying any specific sources in your Podfile, the ‘master’
+  spec-repo will be used, as was always the case. However, once you specify specific
+  sources the ‘master’ spec-repo will **not** be included by default. For example:
+
+        source 'private-spec-repo'
+        source 'master'
+
   [François Benaiteau](https://github.com/netbe)
   [#1143](https://github.com/CocoaPods/CocoaPods/pull/1143)
   [Core#19](https://github.com/CocoaPods/Core/pull/19)