doc: specify that maxFreeSockets is per host

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

PR-URL: https://github.com/nodejs/node/pull/40483
Fixes: https://github.com/nodejs/node/issues/40446


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>