doc: correct release notes for non-string map keys in encoding/json
doc: correct release notes for non-string map keys in encoding/json

The original draft mentioned support for json.Marshaler, but that's
not the case.  JSON supports only string keys (not arbitrary JSON)
so only encoding.TextMarshaller is supported.

Change-Id: I7788fc23ac357da88e92aa0ca17b513260840cee
Reviewed-on: https://go-review.googlesource.com/23529
Reviewed-by: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>

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