Document prepared statement close error limitations

[vzer200]

Documents the limitation reported in #185: database/sql discards the driver's statement close error for statements prepared through DB.Prepare or DB.PrepareContext. The comment also identifies directly prepared Tx and Conn statements as paths where the error can be observed. Mock behavior is unchanged.

Validated nine real database/sql preparation/close paths on Go 1.23.12, 1.26.8, and 1.27.1, including Tx.Stmt wrappers and repeated Close calls. The required root go test -race and go vet ./... pass. An additional all-package run encounters an orders-example timeout reproduced on the unchanged base.

Implemented and independently reviewed by automated agents before submission.