Document the evdev "interesting" heuristic

Was checking over this old code, and saw a comment calling me out for a lack of documentation.

It might be half a decade late, but better late then never.

Source/Core/InputCommon/ControllerInterface/evdev/evdev.cpp

@@ -410,8 +410,39 @@ evdevDevice::evdevDevice(const std::string& devnode) : m_devfile(devnode)
 
   // TODO: Add leds as output devices
 
-  // Was there some reasoning behind these numbers?
+  // Filter out interesting devices (see description below)
   m_interesting = num_motion_axis != 0 || num_axis >= 2 || num_buttons >= 8;
+    
+  // On modern linux systems, there are a lot of event devices that aren't controllers.
+  // For example, the PC Speaker is an event device. Webcams sometimes show up as
+  // event devices. The power button is an event device.
+  
+  // We don't want these showing up in the list of controllers, so we use this
+  // heuristic to filter out anything that doesn't smell like a controller:
+  //
+  // More than two analog axis: 
+  //    Most controllers have at least one stick. This rule will catch all such
+  //    controllers, while ignoring anything with a single axis (like the mouse 
+  //    scroll-wheel)
+  //
+  // --- OR ---
+  //
+  // More than 8 buttons:
+  //     The user might be using a digital only pad such as a NES controller.
+  //     This rule caches such controllers, while eliminating any device with
+  //     only a few buttons, like the power button. Sometimes laptops have devices
+  //     with 5 or 6 special buttons, which is why the threshold is set to 8 to
+  //     match a NES controller.
+  // 
+  // --- OR ---
+  //
+  // Any Motion Axis:
+  //     This rule is to catch any theoretical motion controllers with only a few
+  //     buttons that the user might want to use as a controller. 
+  //
+  // This heuristic is quite loose. The user may still see weird devices showing up
+  // as controllers, but it hopefully shouldn't filter out anything they actually
+  // want to use.  
 }
 
 evdevDevice::~evdevDevice()