specify a default for UseGuardFraction in the man page

(as added in commit f4a63f8eab)

doc/tor.1.txt

@@ -1123,7 +1123,7 @@ The following options are useful only for clients (that is, if
     This torrc option specifies whether clients should use the
     guardfraction information found in the consensus during path
     selection. If it's set to 'auto', clients will do what the
-    UseGuardFraction consensus parameter tells them to do.
+    UseGuardFraction consensus parameter tells them to do. (Default: auto)
 
 [[NumEntryGuards]] **NumEntryGuards** __NUM__::
     If UseEntryGuards is set to 1, we will try to pick a total of NUM routers