Document ADDRMAPSRC_NONE.

src/or/or.h

@@ -3857,7 +3857,9 @@ typedef enum {
    * Tor server that told us what its value was. */
   ADDRMAPSRC_DNS,
 
-  /** DOCDOC */
+  /** No remapping has occurred.  This isn't a possible value for an
+   * addrmap_entry_t; it's used as a null value when we need to answer "Why
+   * did this remapping happen." */
   ADDRMAPSRC_NONE
 } addressmap_entry_source_t;