notmuch.el: Make documentation of notmuch-search-mode dynamic.

notmuch.el: Make documentation of notmuch-search-mode dynamic.

Previously, we had some hard-coded keybindings mentioned in the
introductory paragraphs of the documentation for notmuch-search-mode.
Now, we take advantage of the substitute-command-keys functionality to
produce the same text by default, but to dynamically generate the
correct text in the face of the user customizing the keybindings.