Document optional user attributes for SDKs (#15833)

Author: cleptric

develop-docs/sdk/telemetry/logs.mdx

@@ -318,6 +318,8 @@ Logs can be generated in three ways:
 
 #### User Attributes
 
+SDKs may optionally attach user information as attributes (guarded by `sendDefaultPii`):
+
 1. `user.id`: The user ID. Maps to `id` in the [User](/sdk/data-model/event-payloads/user/) payload.
 2. `user.name`: The username. Maps to `username` in the [User](/sdk/data-model/event-payloads/user/) payload.
 3. `user.email`: The email address. Maps to `email` in the [User](/sdk/data-model/event-payloads/user/) payload.