go/parser: document that parse functions need valid token.FileSet

go/parser: document that parse functions need valid token.FileSet

+ panic with explicit error if no file set it provided

(Not providing a file set is invalid use of the API; panic
is the appropriate action rather than returning an error.)

Fixes #16018.

Change-Id: I207f5b2a2e318d65826bdd9522fce46d614c24ee
Reviewed-on: https://go-review.googlesource.com/24010
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>