[docs] Add note to `supportedsites.md` (#12382)

Authored by: seproDev

devscripts/make_supportedsites.py

@@ -10,10 +10,21 @@ sys.path.insert(0, os.path.dirname(os.path.dirname(os.path.abspath(__file__))))
 from devscripts.utils import get_filename_args, write_file
 from yt_dlp.extractor import list_extractor_classes
 
+TEMPLATE = '''\
+# Supported sites
+
+Below is a list of all extractors that are currently included with yt-dlp.
+If a site is not listed here, it might still be supported by yt-dlp's embed extraction or generic extractor.
+Not all sites listed here are guaranteed to work; websites are constantly changing and sometimes this breaks yt-dlp's support for them.
+The only reliable way to check if a site is supported is to try it.
+
+{ie_list}
+'''
+
 
 def main():
     out = '\n'.join(ie.description() for ie in list_extractor_classes() if ie.IE_DESC is not False)
-    write_file(get_filename_args(), f'# Supported sites\n{out}\n')
+    write_file(get_filename_args(), TEMPLATE.format(ie_list=out))
 
 
 if __name__ == '__main__':