Add additional interactive help commands

[xantoz]

This adds describe-key-briefly, which works like its emacs counterpart. It is also bound to C-h c like its emacs counterpart. This forces us to move describe-command to C-h f, but I consider this binding better, since it aligns to describe-function from emacs bound on C-h f.

The second commit adds where-is bound to C-h w which works mostly like its emacs counterpart, except it always searches all keymaps and displays what keymap the binding is from.

[xantoz]

Updated with updated documentation.

[xantoz]

Wrapped a too long line.

[xantoz]

Updated the changelog