Doc: Fix Standalone Performance Issue (OPENIG-4715)

The main required change will be to add that the ClientHandler (and ReverseProxyHandler) connections attribute, when used in Standalone, is not really the absolute maximum.

In fact it is the maximum number of HTTP/1.1 connections to keep in the pool per Vert.x HTTP Client, and we have as many Vert.x HTTP Clients than gatewayUnits (in the admin.json).

So, if you do have set gatewayUnits: 3, you can observe as much as 3 * connections concurrently to the protected application (if this handler is only used for contacting the protected app).