How do I call Net::HTTP::Persistent.shutdown?

Hello. I want to reuse a Net::HTTP connection. However, I want to destroy (close) the connection after one sequence operation.

### sample code
```ruby
conn = Faraday.new do |f|
f.adapter :net_http_persistent, pool_size: 5 do |http|
http.idle_timeout = 5
end
end
conn.get('http://example.com/1')
conn.get('http://example.com/2') # reuse connection
conn.get('http://example.com/3') # reuse connection, end of sequence

conn.close # I want to destroy connection pool

# create new connection pool
conn = Faraday.new do |f|
f.adapter :net_http_persistent, pool_size: 5 do |http|
http.idle_timeout = 5
end
end
conn.get('http://example.com/1')
conn.get('http://example.com/2') # reuse connection
conn.get('http://example.com/3') # reuse connection, end of sequence

conn.close # I want to destroy connection pool
```

I think the `conn.close` method is just calling `Faraday::Adapter::NetHttpPersistent.close`. And `Faraday::Adapter::NetHttpPersistent.close` is not implemented.

I think calling `@cached_connection.shutdown` is destory connections, Like this.
```ruby
conn.app.instance_variable_get(:@cached_connection).shutdown
```

Is there an easier way to destory a connection? Or is it a process that does not require destroying the connection?

Start by tracing Faraday::Adapter::NetHttpPersistent.close and the @cached_connection.shutdown call shown in the issue. Confirm the supported connection-pool lifecycle and document whether callers should close or reuse the pool, including the behavior after shutdown.

Written by the indexing model from the issue text.