Improve description of tracking (U)ID

[CPBridge]

Addresses #459

The tracking ID is already described in SR and SEG pages of the User Guide, but this PR improves the docstrings in the API reference to make it clearer that tracking IDs are used to track ROIs across imaging studies.

[fedorov]

Maybe it also makes sense to add an example somewhere? "Track segment across multiple studies" is a lot of terms that may not be very familiar. So, for example, we could say "As an example, segmentation of a lesion monitored for progression can be assigned the same TrackingUID, allowing to link and monitor segmentaton-derived tumor volume changes over time. As another example, TrackingUID and link segmentations of the same lesion in different imaging modalities within the same imaging study."

[CPBridge]

Thanks @fedorov I adapted your suggestion to improve the user guide page on segmentations