Please simplify the treegrid example and add more details to the treegrid docs

[snoopy72]

Description

The examples of the "treegrid" plugin are quite difficult to understand for non-chinese-sign-readers.

Due the the (very) small documentation, it is (from my point of view) difficult to run the own table with all the fields and values. it is not quite clear, which field is responsible for which relation.

It would save hugh amount of developers time to bring in some more details to such an interesting feature / plugin.