Expand documentation for extension functions (#880)

This relates to #870; it's not immediately clear that you need to pass a
different driver name to sql.Open from the documentation.

doc.go

@@ -93,19 +93,32 @@ you need to set ConnectHook and get the SQLiteConn.
 
 Go SQlite3 Extensions
 
-If you want to register Go functions as SQLite extension functions,
-call RegisterFunction from ConnectHook.
+If you want to register Go functions as SQLite extension functions
+you can make a custom driver by calling RegisterFunction from
+ConnectHook.
 
 	regex = func(re, s string) (bool, error) {
 		return regexp.MatchString(re, s)
 	}
-	sql.Register("sqlite3_with_go_func",
+	sql.Register("sqlite3_extended",
 			&sqlite3.SQLiteDriver{
 					ConnectHook: func(conn *sqlite3.SQLiteConn) error {
 						return conn.RegisterFunc("regexp", regex, true)
 					},
 			})
 
+You can then use the custom driver by passing its name to sql.Open.
+
+	var i int
+	conn, err := sql.Open("sqlite3_extended", "./foo.db")
+	if err != nil {
+		panic(err)
+	}
+	err = db.QueryRow(`SELECT regexp("foo.*", "seafood")`).Scan(&i)
+	if err != nil {
+		panic(err)
+	}
+
 See the documentation of RegisterFunc for more details.
 
 */