Write a bunch of module documentation.

This commit adds or improves the module-level documenation for:

  buffers.c circuitstats.c command.c connection_edge.c control.c
  cpuworker.c crypto_curve25519.c crypto_curve25519.h
  crypto_ed25519.c crypto_format.c dircollate.c dirserv.c dns.c
  dns_structs.h fp_pair.c geoip.c hibernate.c keypin.c ntmain.c
  onion.c onion_fast.c onion_ntor.c onion_tap.c periodic.c
  protover.c protover.h reasons.c rephist.c replaycache.c
  routerlist.c routerparse.c routerset.c statefile.c status.c
  tor_main.c workqueue.c

In particular, I've tried to explain (for each documented module)
what each module does, what's in it, what the big idea is, why it
belongs in Tor, and who calls it.  In a few cases, I've added TODO
notes about refactoring opportunities.

I've also renamed an argument, and fixed a few DOCDOC comments.

src/common/workqueue.c

@@ -6,6 +6,20 @@
  *
  * \brief Implements worker threads, queues of work for them, and mechanisms
  * for them to send answers back to the main thread.
+ *
+ * The main structure here is a threadpool_t : it manages a set of worker
+ * threads, a queue of pending work, and a reply queue.  Every piece of work
+ * is a workqueue_entry_t, containing data to process and a function to
+ * process it with.
+ *
+ * The main thread informs the worker threads of pending work by using a
+ * condition variable.  The workers inform the main process of completed work
+ * by using an alert_sockets_t object, as implemented in compat_threads.c.
+ *
+ * The main thread can also queue an "update" that will be handled by all the
+ * workers.  This is useful for updating state that all the workers share.
+ *
+ * In Tor today, there is currently only one thread pool, used in cpuworker.c.
  */
 
 #include "orconfig.h"