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/or/command.c

@@ -7,6 +7,26 @@
 /**
  * \file command.c
  * \brief Functions for processing incoming cells.
+ *
+ * When we receive a cell from a client or a relay, it arrives on some
+ * channel, and tells us what to do with it. In this module, we dispatch based
+ * on the cell type using the functions command_process_cell() and
+ * command_process_var_cell(), and deal with the cell accordingly.  (These
+ * handlers are installed on a channel with the command_setup_channel()
+ * function.)
+ *
+ * Channels have a chance to handle some cell types on their own before they
+ * are ever passed here --- typically, they do this for cells that are
+ * specific to a given channel type.  For example, in channeltls.c, the cells
+ * for the initial connection handshake are handled before we get here.  (Of
+ * course, the fact that there _is_ only one channel type for now means that
+ * we may have gotten the factoring wrong here.)
+ *
+ * Handling other cell types is mainly farmed off to other modules, after
+ * initial sanity-checking.  CREATE* cells are handled ultimately in onion.c,
+ * CREATED* cells trigger circuit creation in circuitbuild.c, DESTROY cells
+ * are handled here (since they're simple), and RELAY cells, in all their
+ * complexity, are passed off to relay.c.
  **/
 
 /* In-points to command.c: