crypto/x509: document AuthorityKeyId and don't mutate it.
crypto/x509: document AuthorityKeyId and don't mutate it.

The AuthorityKeyId value from the template was used by
CreateCertificate, but that wasn't documented. Also, CreateCertificate
would stash a value in the template if it needed to override it, which
was wrong: template should be read-only.

Fixes #18962.

Change-Id: Ida15c54c341e5bbf553756e8aa65021d8085f453
Reviewed-on: https://go-review.googlesource.com/36556
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
Run-TryBot: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

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