Update mwjs.rst to say you don't need a . in mw.exec() str's

  them and displays them locally.

`mw.exec(str)`

  Takes a string argument and executes it as a talker command.

  Takes a string argument and executes it as a talker command. Preceeding '.' not needed.

`mw.say(str)`

  Takes an argument which it sends as a message in the talker.