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 <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

ImJasonH/go,josharian/go,theif519/go,josharian/go,Cofyc/go,goodlang/good,linux-on-ibm-z/go,suncycheng/go,nawawi/go,akutz/go,goodlang/good,vsekhar/elastic-go,codestation/go,ImJasonH/go,golang/go,derekmarcotte/go,akutz/go,theif519/go,4ad/go,suncycheng/go,4ad/go,ImJasonH/go,nawawi/go,codestation/go,theif519/go,linux-on-ibm-z/go,Cofyc/go,josharian/go,linux-on-ibm-z/go,golang/go,CAFxX/go,josharian/go,golang/go,cloudflare/go,codestation/go,CAFxX/go,theif519/go,derekmarcotte/go,akutz/go,CAFxX/go,nawawi/go,nawawi/go,akutz/go,linux-on-ibm-z/go,theif519/go,momchil-velikov/go,derekmarcotte/go,codestation/go,golang/go,akutz/go,nawawi/go,polyverse-security/go,Cofyc/go,Cofyc/go,polyverse-security/go,CAFxX/go,goodlang/good,suncycheng/go,linux-on-ibm-z/go,golang/go,theif519/go,ImJasonH/go,suncycheng/go,ImJasonH/go,4ad/go,vsekhar/elastic-go,goodlang/good,vsekhar/elastic-go,ImJasonH/go,polyverse-security/go,momchil-velikov/go,cloudflare/go,suncycheng/go,codestation/go,Cofyc/go,derekmarcotte/go,cloudflare/go,4ad/go,linux-on-ibm-z/go,derekmarcotte/go,akutz/go,4ad/go,Cofyc/go,cloudflare/go,momchil-velikov/go,momchil-velikov/go,CAFxX/go,golang/go,cloudflare/go,linux-on-ibm-z/go,nawawi/go,ImJasonH/go,momchil-velikov/go,vsekhar/elastic-go,goodlang/good,momchil-velikov/go,theif519/go,momchil-velikov/go,CAFxX/go,derekmarcotte/go,akutz/go,suncycheng/go,polyverse-security/go,golang/go,vsekhar/elastic-go,goodlang/good,vsekhar/elastic-go,josharian/go,vsekhar/elastic-go,codestation/go,polyverse-security/go,derekmarcotte/go,golang/go,cloudflare/go,christopher-henderson/Go,Cofyc/go,CAFxX/go,CAFxX/go,theif519/go,goodlang/good,suncycheng/go,Cofyc/go,akutz/go,josharian/go,ImJasonH/go,polyverse-security/go,4ad/go,nawawi/go,polyverse-security/go,codestation/go,codestation/go,derekmarcotte/go,cloudflare/go,josharian/go,nawawi/go,josharian/go,vsekhar/elastic-go,linux-on-ibm-z/go,cloudflare/go,polyverse-security/go,4ad/go,momchil-velikov/go,suncycheng/go,goodlang/good,4ad/go