prefer std::string for setComment()

in case of embedded nulls
diff --git a/include/json/value.h b/include/json/value.h
index d91ccc4..26bb38b 100644
--- a/include/json/value.h
+++ b/include/json/value.h
@@ -512,6 +512,7 @@
   //# endif
 
   /// \deprecated Always pass len.
+  JSONCPP_DEPRECATED("Use setComment(std::string const&) instead.")
   void setComment(const char* comment, CommentPlacement placement);
   /// Comments must be //... or /* ... */
   void setComment(const char* comment, size_t len, CommentPlacement placement);