doc: clarify sub-1000ms behavior in socket.setKeepAlive

The current documentation states that `initialDelay` and `interval`
are divided by 1000 and rounded down, but does not explicitly mention
that values less than 1000ms will result in 0 seconds. This can lead
to confusion when users pass values like 400ms expecting sub-second
keepalive delays.

Add a concrete example showing that 400ms rounds down to 0 seconds.

Refs: #57712

Signed-off-by: haramjeong <04harams77@gmail.com>
PR-URL: #65869
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ulises Gascón <ulisesgascongonzalez@gmail.com>

Author: haramj

doc/api/net.md

@@ -1628,7 +1628,9 @@ corresponding system default unchanged.
 
 `initialDelay` and `interval` are specified in milliseconds but the
 underlying socket options are configured in whole seconds; the values are
-divided by `1000` and rounded down before being applied.
+divided by `1000` and rounded down before being applied. For example,
+setting `initialDelay` to `400` will result in a `TCP_KEEPIDLE` of `0`
+seconds (since `400 / 1000` rounds down to `0`).
 
 Enabling the keep-alive functionality will set the following socket options: