many lines of blabber at top of *Completions* buffer

[Dan Jacobson]

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

   Possible completions are:
   ../                             ./
   MAKEFLOPPIES                    a2dismod
   a2dissite                       a2enmod
   a2ensite                        ab...

Hurmf, wasted 1, 2, 3, 4 or 5 lines of precious completion buffer
space, depending on one's experience.  Might as well throw in a total
count of how many completions, like bash if lots. And mention the
variables to limit the above verboseness.