Merge pull request #378 from Kapps/fix11001

Fix issue 11001 - Add documentation for __traits(getVirtualIndex).

traits.dd

@@ -45,6 +45,7 @@ $(GNAME TraitsKeyword):
     $(GBLINK getVirtualMethods)
     $(GBLINK parent)
     $(GBLINK classInstanceSize)
+    $(GBLINK getVirtualIndex)
     $(GBLINK allMembers)
     $(GBLINK derivedMembers)
     $(GBLINK isSame)
@@ -538,6 +539,15 @@ $(H2 $(GNAME classInstanceSize))
 	polymorphic type.
 	)
 
+$(H2 $(GNAME getVirtualIndex))
+
+  $(P Takes a single argument which must evaluate to a function.
+  The result is a $(CODE ptrdiff_t) containing the index
+  of that function within the vtable of the parent type. 
+  If the function passed in is final and does not override
+  a virtual function, $(B -1) is returned instead.
+  )
+
 $(H2 $(GNAME allMembers))
 
 	$(P Takes a single argument, which must evaluate to either