Document the standard [incr tsdb()] schema

[goodmami]

@janmbuys suggested that it would be helpful for the PyDelphin documentation to describe the relation and fields in the standard [incr tsdb()] schema. Someone not already familiar with [incr tsdb()] would not know where to get the data relevant for their task.

[goodmami]

This would be better documented in the newer https://github.com/delph-in/docs/ repository, although PyDelphin would do well to link to this page, once created (see delph-in/docs#20).