Changes to make generated script docs understand new policy/ hierarchy.

Added an arg to the search_for_files() util function that can return
the subpath of BROPATH's policy/ dir in which the loaded file is found.
This subpath is then used in both the the reST file's document title
(so that script's named e.g. "base.bro" actually have some context) and
in figuring out how to interlink with other generated docs of other
scripts that are found in @load directives.

I still need to overhaul things so the loading of "packages" is
documented in a meaningful way and that the CMake targets are able
to generate indexes for packages.

src/BroDoc.h

@@ -24,9 +24,11 @@ public:
 	 * If the filename doesn't end in ".bro", then ".rst" is just appended.
 	 * Any '/' characters in the reST file name that result from choice of
 	 * the 'rel' parameter are replaced with '^'.
-	 * @param rel A string representing the path relative to BROPATH off of
-	 *        which the source file is loaded or generally any filesystem
-	 *        path to a Bro script.  May or may not have .bro file extension.
+	 * @param subpath A string representing a subpath of BROPATH's policy/
+	 *        directory in which the source file is located.  It can
+	 *        also be full path to the file or a full path that's in BROPATH,
+	 *        but in either of those cases, the parameter is essentially
+	 *        ignored and the document title is just derived from file name
 	 * @param abs The absolute path to the Bro script for which to generate
 	 *        documentation.
 	 */