Update documentation for coap-client -O option

examples/client.c
man/coap-client.txt.in

Update information for the -O option to indicate that a leading 0x causes
the hex text to be included as binary data.

examples/client.c

@@ -648,7 +648,9 @@ usage( const char *program, const char *version) {
      "\t       \t\t(default is %d)\n"
      "\t-K interval\tsend a ping after interval seconds of inactivity\n"
      "\t-N     \t\tSend NON-confirmable message\n"
-     "\t-O num,text\tAdd option num with contents text to request\n"
+     "\t-O num,text\tAdd option num with contents text to request. If the\n"
+     "\t       \t\ttext begins with 0x, then the hex text is converted to\n"
+     "\t       \t\tbinary data\n"
      "\t-P addr[:port]\tUse proxy (automatically adds Proxy-Uri option to\n"
      "\t       \t\trequest)\n"
      "\t-T token\tInclude specified token\n"

man/coap-client.txt.in

@@ -117,7 +117,8 @@ OPTIONS - General
    confirmable message will be sent.
 
 *-O* num,text::
-   Add option 'num' with contents of 'text' to the request.
+   Add option 'num' with contents of 'text' to the request. If the text begins
+   with 0x, then the hex text is converted to binary data.
 
 *-P* addr[:port]::
    Address (and port) for proxy to use (automatically adds Proxy-Uri option