Made the spec more clear

--HG--
extra : convert_revision : svn%3Afdd8eb12-d10e-0410-9acb-85c331704f74/trunk%402635

dlib/threads/thread_pool_extension_abstract.h

@@ -15,7 +15,7 @@ namespace dlib
     {
         /*!
             WHAT THIS OBJECT REPRESENTS
-                This object represents a group of threads which you can
+                This object represents a fixed size group of threads which you can
                 submit tasks to and then wait for those tasks to be completed. 
         !*/