clarify custom unit

Author: Lms24

develop-docs/sdk/telemetry/scopes.mdx

@@ -63,7 +63,7 @@ Attributes are key-value pairs where each value is either a an attribute value o
 - `value`: The actual attribute value, which MUST match the specified type
 - `unit` (optional): The unit of measurement (e.g., `"millisecond"`, `"second"`, `"byte"`, `"ration"`).
   - SDKs MAY delay adding support for units for the moment, but MUST be able to do so at a later date without a breaking change.
-  - SDKS MUST allow users to set all units supported by Relay's [`MetricUnit` enum](https://getsentry.github.io/relay/relay_metrics/enum.MetricUnit.html) but MUST NOT allow custom units [(`CustomUnit`)](https://getsentry.github.io/relay/relay_metrics/struct.CustomUnit.html) or "none" units (`''` or `'none'`).
+  - SDKS MUST allow users to set all units supported by Relay's [`MetricUnit` enum](https://getsentry.github.io/relay/relay_metrics/enum.MetricUnit.html) but MUST NOT allow custom units (i.e. arbitrary strings, defined in [`CustomUnit`](https://getsentry.github.io/relay/relay_metrics/struct.CustomUnit.html)) or "none" units (`''` or `'none'`).
 - `type` (optional): The type of the attribute. SDKs SHOULD NOT expose this to users if they can reliably infer the type from the value. If not, SDKs MAY allow or require users to set the `type` explicitly.
 
 #### Example Usage