database/sql: clarify behavior of DB.Close

It wasn't clear for existing docs if DB.Close forcefully closed
connections or waited for them to finish.

Fixes #23753

Change-Id: Id7df31224c93181c8d01bab7b0b23da25b42a288
Reviewed-on: https://go-review.googlesource.com/103397Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/database/sql/sql.go

@@ -738,7 +738,9 @@ func (db *DB) Ping() error {
 	return db.PingContext(context.Background())
 }
 
-// Close closes the database, releasing any open resources.
+// Close closes the database and prevents new queries from starting.
+// Close then waits for all queries that have started processing on the server
+// to finish.
 //
 // It is rare to Close a DB, as the DB handle is meant to be
 // long-lived and shared between many goroutines.