made specs more clear

--HG--
extra : convert_revision : svn%3Afdd8eb12-d10e-0410-9acb-85c331704f74/trunk%403608

dlib/manifold_regularization/graph_creation_abstract.h

@@ -33,7 +33,7 @@ namespace dlib
             - num > 0
             - random_seed must be convertible to a string by dlib::cast_to_string()
             - dist_funct(samples[i], samples[j]) must be a valid expression that evaluates
-              to a floating point number
+              to a floating point number >= 0
         ensures
             - This function randomly samples the space of pairs of integers between
               0 and samples.size()-1 inclusive.  For each of these pairs, (i,j), a
@@ -68,7 +68,7 @@ namespace dlib
             - samples.size() > k
             - k > 0
             - dist_funct(samples[i], samples[j]) must be a valid expression that evaluates
-              to a floating point number
+              to a floating point number >= 0
         ensures
             - #out == a set of sample_pair objects that represent all the k nearest
               neighbors in samples according to the given distance function dist_funct.

dlib/manifold_regularization/sample_pair_abstract.h

@@ -16,7 +16,7 @@ namespace dlib
         /*!
             WHAT THIS OBJECT REPRESENTS
                 This object is intended to represent an edge in an undirected graph 
-                that has data samples in its vertices.  So it contains two integers
+                which has data samples at its vertices.  So it contains two integers
                 (index1 and index2) which represent the identifying indices of 
                 the samples at the ends of an edge.  Note that this object enforces
                 the constraint that index1 <= index2.  This has the effect of