docs: add proxy agent options docs (#2193)

* add proxy agent options docs

* Apply suggestions from code review

* updated docs to suggested

Author: dancastillo

docs/api/ProxyAgent.md

@@ -19,7 +19,9 @@ Extends: [`AgentOptions`](Agent.md#parameter-agentoptions)
 * **uri** `string` (required) - It can be passed either by a string or a object containing `uri` as string.
 * **token** `string` (optional) - It can be passed by a string of token for authentication.
 * **auth** `string` (**deprecated**) - Use token.
-* **clientFactory** `(origin: URL, opts: Object) => Dispatcher` - Default: `(origin, opts) => new Pool(origin, opts)`
+* **clientFactory** `(origin: URL, opts: Object) => Dispatcher` (optional) - Default: `(origin, opts) => new Pool(origin, opts)`
+* **requestTls** `BuildOptions` (optional) - Options object passed when creating the underlying socket via the connector builder for the request. See [TLS](https://nodejs.org/api/tls.html#tlsconnectoptions-callback).
+* **proxyTls** `BuildOptions` (optional) - Options object passed when creating the underlying socket via the connector builder for the proxy server. See [TLS](https://nodejs.org/api/tls.html#tlsconnectoptions-callback).
 
 Examples: