Add TODO to make Put.recordSizelimit() abstract in the next major

The record size limit is now an implementation detail of the node, so
should be set in the node implementation.

src/dhtproto/node/request/Put.d

@@ -88,6 +88,12 @@ public abstract scope class Put : SingleKey
 
     /***************************************************************************
 
+        TODO: this method should be made abstract, as the record size limit is
+        now intended to be an implementation detail of the node, rather than
+        a constant defined by the protcol. Not making this abstract now in order
+        to avoid a breaking change in a minor branch, but using the magic word
+        "deprecated" so we'll find it when we do the next major release.
+
         Returns:
             the maximum size (in bytes) allowed for a record to be added to the
             storage engine