NGINX command line signal flag (`-s`) docs should document that this flag should only be used on Windows

[alessfg]

Bug Overview

When looking through our Command-line parameters docs (https://nginx.org/en/docs/switches.html), the nginx -s signal parameter does not include any documentation regarding how the flag was developed for usage in Windows platforms and is not recommended on Unix platforms (see nginx/nginx#720 (comment) for more context).

Expected Behavior

We should update our docs (https://nginx.org/en/docs/switches.html) to reflect that the -s flag is only recommended on Windows builds (https://nginx.org/en/docs/windows.html) and link to https://nginx.org/en/docs/control.html#reconfiguration for more info on how to properly reload NGINX. https://nginx.org/en/docs/control.html#reconfiguration could probably me updated too to give a few examples on how to reload the NGINX config on the most popular distros (it tends to involve a service reload).

Steps to Reproduce the Bug

Check https://nginx.org/en/docs/switches.html

Environment Details

Production instance of https://nginx.org/

Additional Context

No response