Fix typo in magit-insert-unpushed-cherries

pulled -> pushed
Merge pull request #984 from mgalgs/typo

Fix typo in magit-insert-unpushed-cherries
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.
For reflog: use the magit-log-mode documentation
Help string for magit-diff-mode.