crypto/x509: clarify accepted keys for MarshalPKCS8PrivateKey
crypto/x509: clarify accepted keys for MarshalPKCS8PrivateKey

Fixes #24413.

Change-Id: I265088c9ddc624cb3b3132087cc3d4baf95d2777
Reviewed-on: https://go-review.googlesource.com/100839
Reviewed-by: Filippo Valsorda <4f40cda291c5f9634e1affd3db44947af61f705c@golang.org>

josharian/go,nawawi/go,josharian/go,codestation/go,codestation/go,cloudflare/go,nawawi/go,josharian/go,codestation/go,Cofyc/go,CAFxX/go,cloudflare/go,golang/go,cloudflare/go,CAFxX/go,nawawi/go,codestation/go,golang/go,nawawi/go,golang/go,CAFxX/go,golang/go,golang/go,nawawi/go,Cofyc/go,nawawi/go,codestation/go,golang/go,cloudflare/go,Cofyc/go,golang/go,akutz/go,codestation/go,CAFxX/go,josharian/go,akutz/go,Cofyc/go,josharian/go,cloudflare/go,Cofyc/go,cloudflare/go,codestation/go,golang/go,josharian/go,akutz/go,akutz/go,CAFxX/go,akutz/go,josharian/go,CAFxX/go,cloudflare/go,nawawi/go,CAFxX/go,Cofyc/go,josharian/go,cloudflare/go,Cofyc/go,nawawi/go,Cofyc/go,akutz/go,CAFxX/go,codestation/go,akutz/go,akutz/go