database/sql: document that drivers may not return right after cancel
database/sql: document that drivers may not return right after cancel

Fixes #18168

Change-Id: Idbfe3d4daedd93c7caf6f1770ecd988e9af39949
Reviewed-on: https://go-review.googlesource.com/34144
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

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