doc: call out http(s).globalAgent default

Despite the `http.Agent` stating:

> The default `http.globalAgent` that is used by `http.request()` has
> all of these values set to their respective defaults.

this isn't true anymore since node.js 19. Both, the http as well as the
https `globalAgent` now set `{ keepAlive: true, scheduling: 'lifo',
timeout: 5000 }` as options. `'lifo'` is the default anyway, but
`keepAlive` is turned off and no `timeout` is set on `new Agent()`.

Document the diverging behaviour in the `globalAgent` sections, remove
the false statement from `http.Agent` section, and extend the changelog
to call out the timeout change as well.

PR-URL: https://github.com/nodejs/node/pull/52392
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Paolo Insogna <paolo@cowtech.it>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Marco Ippolito <marcoippolito54@gmail.com>
Reviewed-By: Ulises Gascón <ulisesgascongonzalez@gmail.com>
Reviewed-By: Moshe Atlow <moshe@atlow.co.il>

doc/api/https.md

@@ -327,10 +327,13 @@ changes:
   - version:
       - v19.0.0
     pr-url: https://github.com/nodejs/node/pull/43522
-    description: The agent now uses HTTP Keep-Alive by default.
+    description: The agent now uses HTTP Keep-Alive and a 5 second timeout by
+                 default.
 -->
 
-Global instance of [`https.Agent`][] for all HTTPS client requests.
+Global instance of [`https.Agent`][] for all HTTPS client requests. Diverges
+from a default [`https.Agent`][] configuration by having `keepAlive` enabled and
+a `timeout` of 5 seconds.
 
 ## `https.request(options[, callback])`