go/ast: fix Object's doc comment about Data

The doc comment about the Data field of go/ast.Object reflects its old
behavior, from when the go/types typechecker depended on ast.Objects.

Since when the doc was written, the behavior has changed in
https://golang.org/cl/7058060 and https://golang.org/cl/7096048 .

Fixes #18631

Change-Id: I10fc3e31cfbf7b303eec44150df917f6eb285f90
Reviewed-on: https://go-review.googlesource.com/35075


Reviewed-by: Robert Griesemer <gri@golang.org>