WL#4078: Document binary format of binlog entries

Documented some binlog events using doxygen. More will be done later.
Also fixed typos in other comments and added remarks about dubious code.
Only comments are affected, there is no change to the actual code.


sql/log_event.cc:
  Fixed typos in some comments.
  Added remarks (as comments) about questionable code.
sql/log_event.h:
  Documented the binary format of following binlog events:
  Log_event
  Query_log_event
  Muted_query_log_event
  Slave_log_event (partial)
  Load_log_event
  Intvar_log_event
  Rand_log_event
  Rotate_log_event (partial)
sql/sql_class.h:
  Fixed typo in comment.

sql/sql_class.h

@@ -1263,14 +1263,16 @@ public:
     We follow this logic:
     - when stmt starts, first_successful_insert_id_in_prev_stmt contains the
     first insert id successfully inserted by the previous stmt.
-    - as stmt makes progress, handler::insert_id_for_cur_row changes; every
-    time get_auto_increment() is called, auto_inc_intervals_for_binlog is
-    augmented with the reserved interval (if statement-based binlogging).
+    - as stmt makes progress, handler::insert_id_for_cur_row changes;
+    every time get_auto_increment() is called,
+    auto_inc_intervals_in_cur_stmt_for_binlog is augmented with the
+    reserved interval (if statement-based binlogging).
     - at first successful insertion of an autogenerated value,
     first_successful_insert_id_in_cur_stmt is set to
     handler::insert_id_for_cur_row.
-    - when stmt goes to binlog, auto_inc_intervals_for_binlog is
-    binlogged if non-empty.
+    - when stmt goes to binlog,
+    auto_inc_intervals_in_cur_stmt_for_binlog is binlogged if
+    non-empty.
     - when stmt ends, first_successful_insert_id_in_prev_stmt is set to
     first_successful_insert_id_in_cur_stmt.
   */