[1329] more documentation update

src/lib/datasrc/sqlite3_accessor.h

@@ -157,12 +157,16 @@ public:
     virtual void deleteRecordInZone(
         const std::string (&params)[DEL_PARAM_COUNT]);
 
+    /// This derived version of the method prepares an SQLite3 statement
+    /// for adding the diff first time it's called, and if it fails throws
+    // an \c SQLite3Error exception.
     virtual void addRecordDiff(
         int zone_id, uint32_t serial, DiffOperation operation,
         const std::string (&params)[DIFF_PARAM_COUNT]);
 
     // A short term method for tests until we implement more complete
-    // API to retrieve diffs.
+    // API to retrieve diffs.  It returns all records of the diffs table
+    // whose zone_id column is identical to the given value.
     std::vector<std::vector<std::string> > getRecordDiff(int zone_id);
 
     /// The SQLite3 implementation of this method returns a string starting