add note about current state of the implementation

doc/Type/Str.pod6

@@ -339,6 +339,11 @@ Directives guide the use (if any) of the arguments. When a directive
 (other than C<%>) is used, it indicates how the next argument
 passed is to be formatted into the string to be created.
 
+NOTE:  The information below is for a fully functioning C<sprintf> implementation which hasn't been
+achieved yet due, in part, to Rakudo issues with native uint64 types. The following section will
+be annotated to indicate what parts do NOT yet wrk properly after a thorough test is made of
+each example below.
+
 The directives are:
 
 =begin table