notmuch.el: Fix notmuch-help to properly display prefixed bindings.

Previously, we would do only a single-level traverse of the keymap.
That meant that for a keybinding such as "M-TAB" we would just see
the prefix key ("ESC") and print that it was a keymap---never printing
the TAB nor the documentation for the command it is bound to.

Now, we do the full walk, constructing a proper description of the
full keybdinding with prefix characters, (and converting "ESC" to
"M-" for legibility).