When using a common type of mouse with two normal buttons, a scroll
wheel, and two side thumb buttons for "forward" and "back", the last
two buttons are not bound in Info-mode.  Patch below.

(As the buttons are not currently bound to anything, I assume that it
is not crucial to make absolutely sure that mouse-6 and mouse-7 have
no other meaning on some other type of mouse.)

Someone might argue that this should instead be part of mwheel.el, and
I have no opinion on that, as long as the buttons work by default.

/Teddy