Document why we must validate file existence

See #1932 (comment)

Author: emmadesilva

packages/framework/src/Support/Filesystem/MediaFile.php

@@ -172,6 +172,7 @@ protected function getNormalizedPath(string $path): string
         // Normalize the path to include the media directory
         $path = static::sourcePath(trim_slashes(Str::after($path, Hyde::getMediaDirectory())));
 
+        // Since assets need to exist on disk in order to be copied to the built site files we validate that the file is real here.
         if (Filesystem::missing($path)) {
             throw new FileNotFoundException($path);
         }