Try to clarify the docs a bit

initially just wanted to fix a typo, but thought maybe the original explanation wasn't great.

Author: caitp

docs/docs/api/ProxyAgent.md

@@ -25,7 +25,7 @@ For detailed information on the parsing process and potential validation errors,
 * **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. It extends from [`Client#ConnectOptions`](/docs/docs/api/Client.md#parameter-connectoptions).
 * **proxyTls** `BuildOptions` (optional) - Options object passed when creating the underlying socket via the connector builder for the proxy server. It extends from [`Client#ConnectOptions`](/docs/docs/api/Client.md#parameter-connectoptions).
-* **proxyTunnel** `boolean` (optional) - By default, ProxyAgent will request that the Proxy facilitate a tunnel between the endpoint and the agent. Setting `proxyTunnel` to false overrides this behaviour for if the proxy and endpoint are both using cleartext http: protocols, similar to the behaviour of Curl.
+* **proxyTunnel** `boolean` (optional) - By default, ProxyAgent will request that the Proxy facilitate a tunnel between the endpoint and the agent. Setting `proxyTunnel` to false avoids issuing a CONNECT extension, and includes the endpoint domain and path in each request.
 
 Examples: