doc: document new TCP_KEEPCNT and TCP_KEEPINTVL socket option defaults

PR https://github.com/nodejs/node/pull/32204 introduced new defaults for
the TCP keep-alive socket options (see deps/uv/src/unix/tcp.c):
- "TCP_KEEPCNT" now defaults to 10 on all platforms
- "TCP_KEEPINTVL" now defaults to 1 on all platforms

Previously, "TCP_KEEPCNT" and "TCP_KEEPINTVL" were not set explicitly
and OS-default values were used.

Fixes: https://github.com/nodejs/node/issues/38298
Refs: https://github.com/nodejs/node/pull/32204

PR-URL: https://github.com/nodejs/node/pull/38313


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>