overwrite-mode docstring should mention yanking

[Dan Jacobson]

  In overwrite mode, printing characters typed in replace existing
  text on a one-for-one basis, rather than pushing it to the right. At
  the end of a line, such characters extend the line. Before a tab,
  such characters insert until the tab is filled in...

Yes however C-y yanking, dabbrev-expand, etc. don't overwrite, so
mention what to do if one wants that to happen too.