notmuch.el: Clean up documentation of notmuch-show-mode-map commands.

As we did recently for notmuch-search-mode-map, ensure that the first
line of docuemntation for each command stands alone.

We also take advantage of the substitute-command-keys functionality
within notmuch-help so that the introductory paragraphs can talk
about key bindings by key (rather than function name) in a way that
will always be current even in the face of the user rebinding keys.