Document varnishlog -w/-r with more details

Fixes #1742

bin/varnishlog/varnishlog_options.h

@@ -38,7 +38,8 @@
 
 #define LOG_OPT_B							\
 	VOPT("B", "[-B]", "Binary output",				\
-	    "Output binary data suitable for reading with -r."		\
+	    "Output binary data suitable for reading with -r. The -w"	\
+	    " option specifies where the binary output is written."	\
 	)
 
 #define LOG_OPT_w							\

include/vut_options.h

@@ -81,7 +81,8 @@
 
 #define VUT_OPT_r							\
 	VOPT("r:", "[-r filename]", "Binary file input",		\
-	    "Read log in binary file format from this file."		\
+	    "Read log in binary file format from this file. The file"   \
+	    " can be created with ``varnishlog -B -w filename``."	\
 	)
 
 #define VUT_OPT_V							\