[Docs] Add explanation on why trait calls are 0 in the API count.

This one is by design and the same in all server side SDKs. Set Traits calls actually call the identity endpoint. It means the usage graph looks odd.

No file, test, or documentation entry point is named. Start by locating the API usage-count and Set Traits documentation in the Swift SDK; explain that Set Traits calls the identity endpoint and therefore appears as zero trait calls in the API usage graph. Done means the design rationale and the graph's unusual appearance are documented clearly.

Written by the indexing model from the issue text.