Expose `(segment id)` region expression. (#1282)

* Expose and document the (segment id) region expressions.
* gen-labels.py was not up to speed with decor #1235.
* The SWC example is read from disk and inserted into the docs verbatim.
* The same file is used to generate the image.
* The missing decor constructor has been added.
* Generating inputs.py on the fly when building docs. That should prevent this issue from coming up again.