Doc: Agent ctor options are not fully documented, such as `lookup`

[bitinn]

ref:

• tls: accept lookup option for tls.connect() #12839

issue:

• Agent ctor does accept options that will be used by socket.connect.
• But I can't find any reference of this at new Agent([options]).
• To be fair, agent.createConnection(options[, callback]) does mention this but few users realize this allows them to make custom dns lookup via passing a custom Agent instance.

[sam-github]

Agreed, if the options are passed to net.connect (EDIT: sorry, socket.connect()), the docs should say so. Can you PR a fix?

[bitinn]

@sam-github I will take a look at doc style guide and see what I can do.

[nanomosfet]

@Trott I can take a spin at this if thats okay. I think we just need to look at the socket.connect() options docs/code and add that into our Agent docs. Does that seem about right?