database/sql: update doc related to per-connection state

Fixes #25330

Change-Id: I41b5b3459ed7808d18112ec02d734e2268099a81
Reviewed-on: https://go-review.googlesource.com/116596Reviewed-by: Daniel Theophanes <kardianos@gmail.com>

src/database/sql/sql.go

@@ -340,8 +340,8 @@ var ErrNoRows = errors.New("sql: no rows in result set")
 //
 // The sql package creates and frees connections automatically; it
 // also maintains a free pool of idle connections. If the database has
-// a concept of per-connection state, such state can only be reliably
-// observed within a transaction. Once DB.Begin is called, the
+// a concept of per-connection state, such state can be reliably observed
+// within a transaction (Tx) or connection (Conn). Once DB.Begin is called, the
 // returned Tx is bound to a single connection. Once Commit or
 // Rollback is called on the transaction, that transaction's
 // connection is returned to DB's idle connection pool. The pool size