cmd/compile: update comments only for Node types and some functions

Improve the comments in syntax.go on Node structs and constants. Also, updated a
few function header comments.

Change-Id: I3e6e4a3c5678fc0b4e18844507b3460303ce1240
Reviewed-on: https://go-review.googlesource.com/c/go/+/269538


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