fmt: clarify that for %g precision determines number of significant digits
fmt: clarify that for %g precision determines number of significant digits

Documentation change only.

Fixes #15178.

Change-Id: I3c7d80ce9e668ac7515f7ebb9da80f3bd8e534d6
Reviewed-on: https://go-review.googlesource.com/22006
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

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