cmd/compile: fmt improvements for AST nodes and some comments on AST nodes

Changed fmt.go to print out some extra information for various kinds of
Nodes. This includes some extra (small) info in the %j (jconv) output,
and some missing sections (such as Dcls and the body of a closure) in
nodedump().

Also, added some extra doc comments for a few Node types in syntax.go

Change-Id: I2ec7184e2abe0d5fbe3fb5a2506da7c7b06f2fb1
Reviewed-on: https://go-review.googlesource.com/c/go/+/266437


Run-TryBot: Dan Scales <danscales@google.com>
Trust: Dan Scales <danscales@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>