Explain Nodes

Hackers,

For my [explanation extension](http://pgxn.org/extension/explanation) I wanted to put together a list of node types,
sinceI'm always having to figure them out to decide which nodes I'm interested in. Reading
src/backend/commands/explain.cI assembled this list: 
   + Aggregate   + Append   + Bitmap Heap Scan   + Bitmap Index Scan   + BitmapAnd   + BitmapOr   + CTE Scan   +
ForeignScan   + Function Scan   + Group   + Hash   + Hash Join   + Index Scan   + Limit   + LockRows   + Materialize
+Merge Append   + Merge Join   + ModifyTable   + Nested Loop   + Recursive Union   + Result   + Seq Scan   + SetOp   +
Sort  + Subquery Scan   + Tid Scan   + Unique   + Values Scan   + WindowAgg   + WorkTable Scan 

Is that accurate? I was looking at how `sname` was set, but maybe it can be set other ways?

Should a list like this be added to the EXPLAIN docs?

Thanks,

David