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

Since notmuch-help now displays a single line of documentation from
each of these commands we ensure that the first line stands alone for
each command.

We also override some builtin commands with new commands that don't
behave any differently, but have our own notmuch-specific
documentation, (such as "select next thread" rather than "move point
to next line").