doc: explain edge case when assigning port to url

numbers which are coerced to scientific notation via .toString(),
will behave unexpectedly when assigned to a url's port.

Fixes: https://github.com/nodejs/node/issues/19595

PR-URL: https://github.com/nodejs/node/pull/19645


Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>