Basically, this prevents Windows from attempting to detect proxy settings for your currently-connected LAN. Despite most modern LANs not using proxies, if you don't use this fix, Windows will, for compatibility reasons, attempt to determine if your LAN requires a proxy every time you go to a new webpage. That's what the "Automatically Detect Settings" checkbox in the LAN Settings of the Connections tab controls, that is, it controls whether or not Windows attempts to detect your LAN's proxy settings (or lack thereof).
If your network happens to use a proxy, performing this fix will break your network connection unless you've manually specified the proxy used...so that's a good reason for a disclaimer note. Otherwise, as already stated, most modern LANs don't use proxies anymore, so this is a safe fix.
Oh, and in my personal experience, it does work as advertised. All my browsers, not just the Steam browser, are significantly faster - and I know because I've been experiencing significant lag for quite a few months now despite the physical connection and hardware on my end being fine, and I haven't been able to pin down the reason why. I've also noticed Steam runs faster in general when it displays web-based content (like Community Hubs, your profile, the store page, etc.), much snappier.
