fix Issue 3777 - size_t is undefined

type.dd

@@ -418,6 +418,15 @@ dg(3);   // call o.member(3)
 -------------------
 )
 
+$(SECTION2 $(D size_t) And $(D ptrdiff_t),
+
+    $(P $(D size_t) is an alias to one of the unsigned integral basic types,
+    and represents a type that is large enough to represent an offset into
+    all addressible memory.)
+
+    $(P $(D ptrdiff_t) is an alias to the signed basic type the same size as $(D size_t).)
+)
+
 )
 
 Macros: