Adding comments to some of the high-level functions in replication.

sql/log_event.h:
  Fixed some mistakes in comments.
sql/repl_failsafe.cc:
  Added comment for show_slave_hosts()
sql/slave.cc:
  Added comment for show_master_info(), handle_slave_[sql|io](), and next_event()
sql/sql_binlog.cc:
  Added @param comment.
sql/sql_lex.h:
  Added comment for st_lex_master_info.
sql/sql_repl.cc:
  Added comments for functions executing a statement:
      PURGE BINARY LOGS
      START SLAVE
      STOP SLAVE
      RESET SLAVE
      CHANGE MASTER
      RESET MASTER
      SHOW BINLOG EVENTS
      SHOW MASTER STATUS
      SHOW BINARY LOGS