ref(measurements): Link to Relay MetricUnit and update wording

src/docs/sdk/event-payloads/properties/measurements.mdx

@@ -27,15 +27,14 @@ Standard measurement keys currently supported are from the following list taken
 ]
 ```
 
-Custom measurements need units to be specified. Units currently supported are durations from the list `["ns" , "ms" , "s"]` taken from [here](https://github.com/getsentry/relay/blob/1e45a8817e45408ecbdbd3f0a679a5f5e4885290/relay-metrics/src/protocol.rs#L14-L21). Note that in the future we will extend the available units (for instance, memory units) but we will not implement a generic user-defined unit interface.
-
+For the well-known measurements listed above, Sentry automatically infers units. Custom measurements need units to be specified, defaulting to `"none"` if missing. The full list of supported units is specified on [Relay's `MetricUnit`](https://getsentry.github.io/relay/relay_metrics/enum.MetricUnit.html). Sentry's event ingestion supports arbitrary custom units, but many SDKs will not expose a generic user-defined unit interface.
 
 ```json
 {
   "measurements": {
     "lcp": { "value": 100 },
     "fp": { "value": 123 },
-    "my.custom.metric": { "value": 456, "unit": "ms" }
+    "my.custom.metric": { "value": 456, "unit": "millisecond" }
   }
 }
 ```