Update hash-map.dd

This update is in reference to <#4245>.

The spec currently says that for classes to be used as keys in associative arrays, they *must* override toHash and opEquals. This is untrue because the default implementations use the address of the class instance for computing the hash and for comparisons.

Author: fieldvoid

spec/hash-map.dd

@@ -81,8 +81,7 @@ $(H2 $(LNAME2 testing_membership, Testing Membership))
 
 $(H2 $(LNAME2 using_classes_as_key, Using Classes as the KeyType))
 
-        $(P Classes can be used as the $(I KeyType). For this to work,
-        the class definition must override the following member functions
+        $(P Classes can be used as the $(I KeyType). The behavior is controlled by the following member functions
         of class $(D Object):)
 
         $(UL
@@ -109,6 +108,8 @@ $(H2 $(LNAME2 using_classes_as_key, Using Classes as the KeyType))
             }
         }
         ---
+        The default implementations of $(D toHash) and $(D opEquals) for classes use $(D is)
+        semantics (the address of the class instance is used to compute the hash, and the address of the class instance is used for comparisons).
 
         $(IMPLEMENTATION_DEFINED
         `opCmp` is not used to check for equality by the