expvar: document that the Var interface's String method should return a valid JSON value.
expvar: document that the Var interface's String method should return a valid JSON value.

Change-Id: If4e740f3dbef4053355542eebdd899b3099d872c
Reviewed-on: https://go-review.googlesource.com/21525
Reviewed-by: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>

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