encoding/json: improve Decode example
encoding/json: improve Decode example

Decoding a JSON message does not touch unspecified or null fields;
always use a new underlying struct to prevent old field values from
sticking around.

Fixes: #14640

Change-Id: Ica78c208ce104e2cdee1d4e92bf58596ea5587c8
Reviewed-on: https://go-review.googlesource.com/23483
Reviewed-by: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>

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