encoding/asn1: document that default:x tag only has meaning with optional tag
encoding/asn1: document that default:x tag only has meaning with optional tag

Fixes #16712.

Change-Id: Ib216059c6c0c952162c19e080dcf3799f0652a8d
Reviewed-on: https://go-review.googlesource.com/32171
Run-TryBot: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Quentin Smith <d6b8e48afb2534b213e391cab43016505747a234@golang.org>

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