fwctl_fetch: Add a comment describing the tool.

I meant to include this in the earlier commit adding fwctl_fetch but
forgot to amend the commit before pushing it.

tools/tools/bhyve/fwctl_fetch.c

@@ -4,6 +4,12 @@
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
+/*
+ * Fetch the value of fwctl nodes from a guest.
+ *
+ * Usage: fwctl_fetch <node>
+ */
+
 #include <sys/param.h>
 #include <err.h>
 #include <fcntl.h>