Clarify which Sybil attack can be mitigated by DP

The scope of this section includes both privacy and robustness Some DP
mechanisms might help, to some extent, with privacy, but obviously it
doesn't help with robustness.

draft-ietf-ppm-dap.md

@@ -2576,7 +2576,8 @@ different ways to address Sybil attacks, such as:
 1. Removing Client-specific metadata on individual reports, such as through the
    use of anonymizing proxies in the upload flow, as described in
    {{anon-proxy}}.
-1. Differential privacy ({{dp}}) can help mitigate Sybil attacks to some extent.
+1. Some mechanisms for differential privacy ({{dp}}) can help mitigate Sybil
+   attacks against privacy to some extent.
 
 ## Batch-selection Attacks {#batch-selection}