Explain when check_node, when check_edge

[filipsch]

There are two distinct ways to walk down the AST: you can navigate to nodes with check_node(), or walk down edged with check_edge(), or you can do a combination of that. For @sumedh10 it wasn't clear what the best way is. A knowledge article should be added to sqlwhat.readthedocs.io to explain what is best in which situation.

Examples for this can be found here

(a bit similar to check_arg by index or name)