man: drop debug_log, update debug

man/pam_u2f.8.txt

@@ -16,15 +16,7 @@ compliant authenticators.
 
 == OPTIONS
 *debug*::
-Enables debug output
-
-*debug_file*::
-Filename to write debugging messages to. **If this file is missing,
-nothing will be logged**. This regular file **has to be created by the
-user** or **must exist and be a regular file** for anything getting
-logged to it. It is not created by pam-u2f on purpose (for security
-considerations). This filename may be alternatively set to "stderr"
-(default), "stdout", or "syslog".
+Enables logging of debug information using the *syslog*(3) module.
 
 *origin*=_origin_::
 Set the relying party ID for the FIDO authentication procedure. If no