Personally i'm more helped by good examples than these syntax diagrams.
Many years ago when learning SQL i was looking at these state diagrams despite using another database. So yes, they should be added! They may not be usefull for anyone SQL who is able to read the Postgresql grammar definition but for someone learning it it‘s helping very much.
Syntax diagram, just like ye olde Pascal syntax poster!
"state diagrams"? Did you mean "syntax diagrams"?
They don't look very accessible to me, but yes, they are pretty.