database/sql: add doc comment for ErrTxDone
database/sql: add doc comment for ErrTxDone

Change-Id: Idffb82cdcba4985954d061bdb021217f47ff4985
Reviewed-on: https://go-review.googlesource.com/29850
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

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