doc: specify that maxFreeSockets is per host

The `maxFreeSockets` options specifies the maximum number of free
sockets per host.

Fixes: #40446

doc/api/http.md

@@ -158,7 +158,7 @@ changes:
     all hosts in total. Each request will use a new socket
     until the maximum is reached.
     **Default:** `Infinity`.
-  * `maxFreeSockets` {number} Maximum number of sockets to leave open
+  * `maxFreeSockets` {number} Maximum number of sockets per host to leave open
     in a free state. Only relevant if `keepAlive` is set to `true`.
     **Default:** `256`.
   * `scheduling` {string} Scheduling strategy to apply when picking