directory-level doxygen for "src/core"

src/core/mainloop/core_mainloop.dox

@@ -1,4 +1,12 @@
 /**
 @dir /core/mainloop
-@brief core/mainloop
+@brief core/mainloop: Non-onion-routing mainloop functionality
+
+This module uses the event-loop code of \refdir{lib/evloop} to implement an
+asynchronous connection-oriented protocol handler.
+
+The layering here is imperfect: the code here was split from \refdir{core/or}
+without refactoring how the two modules call one another.  Probably many
+functions should be moved and refactored.
+
 **/