Docstring for magit-commit-mode

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

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.
Docstring for magit-diff-mode.
Docstring for magit-wazzup

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

With description for key bindings to checkout, resetting, merge, delete
and renaming branch.
Merge pull request #1034 from vanicat/t/help-string-for-mode

Try to have a better help system