Clarified spec and also slightly improved usability

dlib/svm/svm_multiclass_linear_trainer.h

@@ -263,7 +263,8 @@ namespace dlib
         )
         {
             learn_nonnegative_weights = value;
-            prior = trained_function_type(); 
+            if (learns_nonnegative_weights)
+                prior = trained_function_type(); 
         }
 
         void set_c (

dlib/svm/svm_multiclass_linear_trainer_abstract.h

@@ -175,6 +175,8 @@ namespace dlib
         /*!
             ensures
                 - #learns_nonnegative_weights() == value
+                - if (value == true) then
+                    - #has_prior() == false
         !*/
 
         void set_prior (