bug#7641: use more familiar left-mouse... names

[jidanni]

I think the general public are more familiar with right-mouse
center-mouse left-mouse than these names.

Maybe the could be made into aliases to show the public.

Maybe it would work no matter if the user has configured the mouse for
right handers or left.

      Click <mouse-2> on a completion to select it.
      In this buffer, type RET to select the completion near point.