Bind ? to magit-key-mode-popup-dispatch

magit-describe-item is a debug tool mainly useful for those writing
magit, when magit-key-mode-popup-dispatch is a useful entry point for
a lot of magit function.
Doc string for magit-commit-mode

This mode do mostly nothing, so the help string say not much.
magit-status-mode docstring

A lot to say here. So we just say the minimal, and refer to the manual.
Docstring for magit-log-mode

There is a lot one can do here. So the help string only show a selection
of keybindings.
Docstring for reflog-mode.

We use it for mainly for resetting head.
Help string for magit-diff-mode.
magit-wazzup docstring

Used to look at change, and then cherry-pick or merge them.
docstring for magit-cherry-mode.