go/ast: document that (*ast.File).Comments is sorted by position

This is the way that go/parser produces it and in every AST manipulation
that we do, we preserve such property. Also printer expects it is such
order.

Change-Id: I6a6a696424a679a2cf1f9bb7eb41bdd47523efa9
Reviewed-on: https://go-review.googlesource.com/c/go/+/704255
Reviewed-by: Michael Knyszek <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Alan Donovan <[email protected]>
Auto-Submit: Alan Donovan <[email protected]>

Author: mateusz834

src/go/ast/ast.go

@@ -1064,7 +1064,7 @@ type File struct {
 	Scope              *Scope          // package scope (this file only). Deprecated: see Object
 	Imports            []*ImportSpec   // imports in this file
 	Unresolved         []*Ident        // unresolved identifiers in this file. Deprecated: see Object
-	Comments           []*CommentGroup // list of all comments in the source file
+	Comments           []*CommentGroup // comments in the file, in lexical order
 	GoVersion          string          // minimum Go version required by //go:build or // +build directives
 }