[privacy_budget] Mention that we need unique UseCounters.

UseCounter names may be reused for multiple APIs. However for the
purposes of the identifiability study, we need to keep samples from
different APIs in distinct buckets.

Bug: 973801
Change-Id: I4ee69e6c5317a177e636f4627fa1326aa9a180e5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2426725
Auto-Submit: Asanka Herath <asanka@chromium.org>
Reviewed-by: Dylan Cutler <dylancutler@google.com>
Commit-Queue: Dylan Cutler <dylancutler@google.com>
Cr-Commit-Position: refs/heads/master@{#809818}

docs/privacy_budget_instrumentation.md

@@ -326,6 +326,12 @@ interface MyInterface : EventTarget {
 There's no hard rule about this, but the `UseCounter` name is an implementation
 detail that doesn't belong in the IDL. It also adds unnecessary noise.
 
+*** note
+**IMPORTANT** Make sure that each API has its own `UseCounter` name. Otherwise
+multiple APIs will have their samples accumulate within the same bucket. This
+alters the observed characteristics of the API from what it really is.
+***
+
 <!-- Sort (case insensitive), but don't line-wrap -->
 [`blink::Document`]: ../third_party/blink/renderer/core/dom/document.h
 [`blink::ExecutionContext`]: ../third_party/blink/renderer/core/execution_context/execution_context.h