update example code to make ReferenceFileDocument sendable

[heckj]

latest Xcode (16) and associated SDKs have marked ReferenceFileDocument as Sendable. I inquired in the developer forums about this, and the DTS engineer kindly explained that it has to be Sendable - which means adding some structure in there - because the various methods are called from a number of different isolation domains.

The gist is that the class holds ~3-4 properties that are all possibly mutating, and all of those need to be locked down to Sendable, so wrapped within an OSUnfairAllocatedLock (as per updated ReferenceFileDocument code example).