doc: add documentation for request.path

The field has been added in v0.4.0.

PR-URL: nodejs#25788
Refs: nodejs@b09c588#diff-1c0f1c434b17b7f8795d44a51a14320aR814
Refs: https://github.com/nodejs/node/blob/380929ec0c4c4004b522bed5e3800ebce2b68bfd/test/parallel/test-http-client-defaults.js
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

doc/api/http.md

@@ -633,6 +633,13 @@ const cookie = request.getHeader('Cookie');
 
 Limits maximum response headers count. If set to 0, no limit will be applied.
 
+### request.path
+<!-- YAML
+added: v0.4.0
+-->
+
+* {string} The request path. Read-only.
+
 ### request.removeHeader(name)
 <!-- YAML
 added: v1.6.0