Take stock of the wheel-up/down confusion

Take stock of the wheel-up/down confusion

While we're hopefully all aware of the usual confusion between
the scroll operation moving the document or moving the viewport, Emacs
has its very own instance of that confusion where the
`mouse-wheel-down-event` variable is the one that (used to) hold
the value `wheel-up` and vice versa.

Thanks for Po Lu's commit 957b4f826a4 which not only fixed my change
but brought that confusion to my attention.
This patch doesn't fix the problem, but tries to fix the other places
in the code where we did not take it into account.

* doc/lispref/commands.texi (Misc Events): Mention the
wheel-up/down confusion.

* lisp/progmodes/flymake.el (flymake--mode-line-counter-map):
* lisp/completion-preview.el (completion-preview--mouse-map):
Fix wheel-up/down confusion.

* lisp/mwheel.el (mouse-wheel-down-event, mouse-wheel-up-event):
Fix docstrings.