document -log_dir, the one undocumented flag in this package.

Author: robpike

glog.go

@@ -45,6 +45,9 @@
 //	-stderrthreshold=ERROR
 //		Log events at or above this severity are logged to standard
 //		error as well as to files.
+//	-log_dir=""
+//		Log files will be written to this directory instead of the
+//		default temporary directory.
 //
 //	Other flags provide aids to debugging.
 //