Explain Nodes

Hackers,

For my [explanation extension](http://pgxn.org/extension/explanation) I wanted to put together a list of node types, since I'm always having to figure them out to decide which nodes I'm interested in. Reading src/backend/commands/explain.c I assembled this list:

+ Aggregate
+ Append
+ Bitmap Heap Scan
+ Bitmap Index Scan
+ BitmapAnd
+ BitmapOr
+ CTE Scan
+ Foreign Scan
+ 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