Add documentation for closing a client connection

[karlbecker]

I struggled to find any reference to the close() function in the JavaScript library's documentation, so I thought adding it here would be helpful to many people.

I might be getting some of the terminology wrong, so please feel free to clarify / improve what I've written. Happy to answer any questions you have or redo this if there's a different file I should be editing.

Thanks!

[cla-checker-service]

❌ Author of the following commits did not sign a Contributor Agreement:
57e18a1, d18f098

Please, read and sign the above mentioned agreement if you want to contribute to this project

[karlbecker]

I have not signed the contributor agreement. However, I think it would be very beneficial for the docs to mention how to close a connection, since I haven't seen anything like this in the docs.

If you want to close this PR, that's fine, but could someone with Elastic open up a different PR to make sure documentation of close() gets added to the docs? It would be really helpful, since the only places I found it were Stack Overflow - https://stackoverflow.com/questions/59439108/elasticsearch-js-client-close - and in an Elasticsearch client changelog: https://www.elastic.co/guide/en/elasticsearch/client/javascript-api/current/changelog-client.html#_calling_client_close_will_reject_new_requests

Thanks!

[JoshMock]

@karlbecker closing in favor of #2104. Basically the same idea, to which I give you credit, but wanted to get it fixed since you hadn't signed the CLA. 🙏