add external links to C++ standard concepts

Documentation/doc/General.txt

@@ -1,13 +1,26 @@
 /// \cgalConcept
 class Assignable {};
+
 /// \cgalConcept
+/// Concept from the \cpp standard.
+/// See http://en.cppreference.com/w/cpp/concept/DefaultConstructible
 class DefaultConstructible {};
+
 /// \cgalConcept
+/// Concept from the \cpp standard.
+/// See http://en.cppreference.com/w/cpp/concept/CopyConstructible
 class CopyConstructible {};
+
 /// \cgalConcept
+/// Concept from the \cpp standard.
+/// See http://en.cppreference.com/w/cpp/concept/EqualityComparable
 class EqualityComparable {};
+
 /// \cgalConcept
+/// Concept from the \cpp standard.
+/// See http://en.cppreference.com/w/cpp/concept/LessThanComparable
 class LessThanComparable {};
+
 /// \cgalConcept
 class AdaptableFunctor {};
 /// \cgalConcept
@@ -16,16 +29,32 @@ class AdaptableUnaryFunction {};
 class AdaptableBinaryFunction {};
 
 /// \cgalConcept
+/// Concept from the \cpp standard.
+/// See http://en.cppreference.com/w/cpp/concept/Iterator
 class Iterator {};
+
 /// \cgalConcept
+/// Concept from the \cpp standard.
 class OutputIterator {};
+
 /// \cgalConcept
+/// Concept from the \cpp standard.
+/// See http://en.cppreference.com/w/cpp/concept/InputIterator
 class InputIterator {};
+
 /// \cgalConcept
+/// Concept from the \cpp standard.
+/// See http://en.cppreference.com/w/cpp/concept/ForwardIterator
 class ForwardIterator {};
+
 /// \cgalConcept
+/// Concept from the \cpp standard.
+/// See http://en.cppreference.com/w/cpp/concept/RandomAccessIterator
 class RandomAccessIterator {};
+
 /// \cgalConcept
+/// Concept from the \cpp standard.
+/// See http://en.cppreference.com/w/cpp/concept/BidirectionalIterator
 class BidirectionalIterator {};
 
 /// \cgalConcept