database/sql: doc cleanup on the DB type

R=golang-dev, adg
CC=golang-dev
https://golang.org/cl/7865044

src/pkg/database/sql/sql.go

@@ -177,15 +177,14 @@ var ErrNoRows = errors.New("sql: no rows in result set")
 // DB is a database handle. It's safe for concurrent use by multiple
 // goroutines.
 //
-// If the underlying database driver has the concept of a connection
-// and per-connection session state, the sql package manages creating
-// and freeing connections automatically, including maintaining a free
-// pool of idle connections. If observing session state is required,
-// either do not share a *DB between multiple concurrent goroutines or
-// create and observe all state only within a transaction. Once
-// DB.Open is called, the returned Tx is bound to a single isolated
-// connection. Once Tx.Commit or Tx.Rollback is called, that
-// connection is returned to DB's idle connection pool.
+// 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
+// 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
+// can be controlled with SetMaxIdleConns.
 type DB struct {
 	driver driver.Driver
 	dsn    string