Document our top-level directories

src/app/app.dox

 /**
 @dir app
-@brief app
+@brief app: top-level entry point for Tor
+
+The "app" directory has Tor's main entry point and configuration logic,
+and is responsible for initializing and managing the other modules in
+Tor.
 **/

src/core/core.dox

 /**
 @dir core
-@brief core
+@brief core: main loop and onion routing functionality
+
+The "core" directory has the central protocols for Tor, which every
+client and relay must implement in order to perform onion routing.
+
 **/

src/feature/feature.dox

 /**
 @dir feature
-@brief feature
+@brief feature: domain-specific modules
+
+The "feature" directory has modules that Tor uses only for a particular
+role or service, such as maintaining/using an onion service, operating as a
+relay or a client, or being a directory authority.
+
 **/

src/lib/lib.dox

 /**
 @dir lib
-@brief lib
+@brief lib: low-level functionality.
+
+The "lib" directory contains low-level functionality, most of it not
+necessarily Tor-specific.
+
 **/

src/tools/tools.dox

 /**
 @dir tools
-@brief tools
+@brief tools: other command-line tools for use with Tor.
+
+The "tools" directory has a few other programs that use Tor, but are not part
+of the main Tor binary.
+
 **/