Improve docs about what needs to be added to file for linkings

[rly]

Hackathon feedback:

If a user creates a Device using the Device constructor and links to it in when creating an ImagingPlane, but the user does not add the Device to the NWBFile, they will get a BrokenLinkError. The documentation should clearly state how Device objects should be created and added to a file. Same thing for ElectrodeGroup, IntracellularElectrode, and many other types.